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Supplemental Prospectus (Form 424B5)

Isabella Bank Corporation

06/16/2026 | Press release | Distributed by Public on 06/16/2026 07:16

Supplemental Prospectus (Form 424B5)


PROSPECTUS SUPPLEMENT						Filed pursuant to Rule 424(b)(5)
(To Prospectus Dated April 23, 2026)						Registration No. 333-295098

$30,000,000

Common Stock

We have entered into an equity distribution agreement (the "Equity Distribution Agreement") with Piper Sandler & Co. (the "Sales Agent" or "Piper Sandler") relating to the sale of shares of our common stock, no par value per share ("common stock"), offered by this prospectus supplement and the accompanying base prospectus. In accordance with the terms of the Equity Distribution Agreement, we may from time to time offer and sell shares of common stock having an aggregate offering price of up to $30.0 million through Piper Sandler, acting as our sales agent.

Sales of our common stock, if any, pursuant to this prospectus supplement and the accompanying base prospectus may be made in transactions that are deemed to be "at the market" offerings as defined in Rule 415(a)(4) promulgated under the Securities Act of 1933, as amended (the "Securities Act"). The Sales Agent is not required to sell any specific amount of securities but will act as sales agent using commercially reasonable efforts to sell on our behalf all of the shares or our common stock requested to be sold by us, consistent with normal trading and sales practices, on mutually agreed terms between the Sales Agent and us. There is no arrangement for funds to be received in any escrow, trust or similar arrangement.

The common stock to which this prospectus supplement relates will be offered and sold at prevailing market prices or at negotiated prices through the Sales Agent over a period of time and from time to time in transactions at then-current prices. The Sales Agent will be entitled to compensation in an amount up to 3.0% of the aggregate gross sales proceeds of any shares of common stock sold under the Equity Distribution Agreement. See "PLAN OF DISTRIBUTION" on page S-13 in this prospectus supplement for additional information regarding compensation to be paid to the Sales Agent. In connection with the sale of common stock on our behalf, the Sales Agent may be deemed to be an "underwriter" within the meaning of the Securities Act, and the compensation of the Sales Agent may be deemed to be underwriting discounts or commissions. We have also agreed to provide indemnification and contribution to the Sales Agent with respect to certain liabilities, including liabilities under the Securities Act.

Our common stock is traded on the Nasdaq Capital Market under the symbol "ISBA." On June 11, 2026, the closing price of our common stock on the Nasdaq Capital Market was $43.24 per share. You are urged to obtain current market prices of our common stock.

Investing in our common stock involves risk. Before making any decision to invest in our common stock, you should carefully consider the risks and uncertainties described in this prospectus supplement under "RISK FACTORS" and the risk factors discussed in our most recent Annual Report on Form 10-K and in our other periodic reports filed with the Securities and Exchange Commission (the "SEC") and incorporated herein by reference.

The common stock offered pursuant to this prospectus supplement has not been approved or disapproved by the SEC, the Federal Deposit Insurance Corporation, the Board of Governors of the Federal Reserve System, or any state securities commission or other regulatory authority nor has the SEC, any state securities commission or other regulatory authority passed upon the accuracy or adequacy of this prospectus. Any representation to the contrary is a criminal offense.

The shares of our common stock are not savings accounts, deposits or other obligations of a bank or savings association and are not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.

The date of this prospectus supplement is June 16, 2026.

TABLE OF CONTENTS

PROSPECTUS SUPPLEMENT

Page


ABOUT THIS PROSPECTUS SUPPLEMENT			S-1
CAUTIONARY NOTE REGARDING FORWARD-LOOKING STATEMENTS			S-1
PROSPECTUS SUPPLEMENT SUMMARY			S-4
Isabella Bank Corporation			S-4
Recent Developments			S-4
Implications of Being a Smaller Reporting Company			S-5
The Offering			S-6
RISK FACTORS			S-7
Risks Associated with this Offering and our Common Stock			S-7
Risks Related to the Grand River Acquisition			S-10
Risks Relating to Our Business			S-12
PLAN OF DISTRIBUTION			S-14
USE OF PROCEEDS			S-16
EXPERTS			S-17
LEGAL MATTERS			S-18
INFORMATION INCORPORATED BY REFERENCE			S-19
WHERE YOU CAN FIND MORE INFORMATION			S-21

BASE PROSPECTUS

Page


ABOUT THIS PROSPECTUS			1
INCORPORATION OF CERTAIN INFORMATION BY REFERENCE			2
WHERE YOU CAN FIND MORE INFORMATION			3
SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS			4
ABOUT ISABELLA BANK CORPORATION			6
RISK FACTORS			7
USE OF PROCEEDS			8
THE SECURITIES WE MAY OFFER			9
DESCRIPTION OF DEBT SECURITIES			10
DESCRIPTION OF COMMON STOCK			23
DESCRIPTION OF SUBSCRIPTION RIGHTS			26
DESCRIPTION OF WARRANTS			28
DESCRIPTION OF UNITS			29
PLAN OF DISTRIBUTION			30
LEGAL MATTERS			33
EXPERTS			34

ABOUT THIS PROSPECTUS SUPPLEMENT

Unless the context otherwise requires, in this prospectus supplement, the terms "Company," "we," "our," "ours," and "us" refer to Isabella Bank Corporation, a financial holding company organized under the laws of Michigan and headquartered in Mount Pleasant, Michigan, and Isabella Bank. The term "Bank" refers to Isabella Bank, our wholly-owned banking subsidiary. The Bank is a Michigan state-chartered commercial bank with banking offices in Michigan.

This document consists of two parts and is part of a registration statement on Form S-3 (the "Registration Statement") that we filed with the Securities and Exchange Commission (the "Commission" or the "SEC") utilizing a "shelf registration" process. The first part of this document is this prospectus supplement, which describes the specific terms of this offering and certain other matters and also adds to and updates information contained in the accompanying base prospectus and the documents incorporated by reference into this prospectus supplement and the accompanying base prospectus. The second part, the accompanying base prospectus, gives more general information about us and the common stock offered by this prospectus supplement and the accompanying base prospectus. Generally, when we refer to the "prospectus", we are referring to this prospectus supplement and the accompanying base prospectus combined as one document. To the extent the information in the prospectus supplement differs from the information in the accompanying base prospectus or any document incorporated by reference filed prior to the date of this prospectus supplement, you should rely on the information in this prospectus supplement.

We are offering to sell, and seeking offers to buy, shares of our common stock only in United States jurisdictions where offers and sales are permitted. We are not making an offer of the common stock in any jurisdiction where the offer is not permitted. The distribution of the prospectus and the offering of the common stock in certain jurisdictions may be restricted by law. The prospectus does not constitute, and may not be used in connection with, an offer to sell, or a solicitation of an offer to buy, any common stock offered by the prospectus by any person in any jurisdiction in which it is unlawful for such person to make such an offer or solicitation.

In making your investment decision, it is important for you to read and consider all of the information contained in this prospectus supplement and the accompanying base prospectus, including the documents incorporated by reference therein. You should rely only on the information contained in, or incorporated by reference in, this prospectus supplement and the accompanying base prospectus. We have not authorized anyone to provide you with information different from that contained in this prospectus. You should not assume that the information that appears in this prospectus supplement, the accompanying base prospectus or any document incorporated by reference into this prospectus supplement or the accompanying base prospectus is accurate as of any date other than their respective dates. Our business, financial condition, results of operations and prospects may have changed since the date of such information.

This prospectus supplement does not contain all of the information included in the Registration Statement. For a more complete understanding of the offering of the securities, you should refer to the Registration Statement, including its exhibits, some of which may be contained in documents subsequently filed with the SEC and incorporated by reference into the prospectus.

You should rely only on the information contained in this prospectus supplement and the accompanying base prospectus. We have not authorized any other person to provide you with information that is different from that contained in, or incorporated by reference in, this prospectus supplement or the accompanying base prospectus. If anyone provides you with different or inconsistent information, you should not rely on it. You should assume that the information appearing in, or incorporated by reference in, this prospectus supplement and the accompanying base prospectus is accurate only as of their respective dates. Our business, financial condition and results of operations may have changed since those dates. This prospectus supplement supersedes the accompanying base prospectus to the extent it contains information that is different from or in addition to the information in that prospectus.

CAUTIONARY NOTE REGARDING FORWARD-LOOKING STATEMENTS

This prospectus supplement, the accompanying base prospectus and the documents incorporated by reference may contain "forward-looking statements" within the meaning of Section 27A of the Securities Act and Section 21E of the Securities Exchange Act of 1934, as amended (the "Exchange Act"). These forward-looking statements reflect our current views with respect to, among other things, future events and our financial performance. These statements are often, but not always, made through the use of words or phrases such as "may," "might," "should," "could,"

S-1

"predict," "potential," "believe," "expect," "continue," "will," "anticipate," "seek," "estimate," "intend," "plan," "strive," "projection," "goal," "target," "outlook," "aim," "would," "annualized" and "outlook," or the negative version of those words or other comparable words or phrases of a future or forward-looking nature. These forward-looking statements are not historical facts, and are based on current expectations, estimates and projections about our industry, management's beliefs and certain assumptions made by management, many of which, by their nature, are inherently uncertain and beyond our control. Accordingly, we caution you that any such forward-looking statements are not guarantees of future performance and are subject to risks, assumptions, estimates and uncertainties that are difficult to predict. Although we believe that the expectations reflected in these forward-looking statements are reasonable as of the date made, actual results may prove to be materially different from the results expressed or implied by the forward-looking statements. When considering forward-looking statements, you should keep in mind the risk factors and other cautionary statements described in the "Risk Factors" section of this prospectus and in the documents incorporated by reference herein, including our most recent Annual Report on Form 10-K.

These factors, many of which are beyond our control, include the following:

•uncertainty or perceived instability in the banking industry as a whole;

•increased competition for deposits among traditional and nontraditional financial services companies, and related changes in deposit customer behavior;

•the lingering inflationary pressures, and the risk of the resurgence of elevated levels of inflation, in the United States and our market areas, and its impact on market interest rates, the labor market, the economy as a whole, and credit quality;

•our ability to effectively execute our expansion strategy and manage our growth, including identifying and consummating suitable acquisitions;

•our ability to consummate our proposed merger with Grand River Commerce, Inc.;

•the ability to obtain required governmental approvals of the proposed terms of the merger;

•risks associated with concentrations of our business in market areas, loans secured by real estate and public funds deposits as a percentage of total deposits;

•adverse changes in customer demand, spending, borrowing, and savings habits;

•risks associated with our commercial loan portfolio and agricultural loan portfolio;

•risks related to the significant amount of credit that we have extended to a limited number of borrowers and in a limited geographic area;

•damage to our reputation resulting from adverse publicity, regulatory actions, litigation, operational failures, and the failure to meet client expectations and other facts;

•our ability to keep pace with technological change or difficulties we may experience when implementing new technologies;

•cybersecurity risk, including cyber incidents or other failures, disruptions or breaches of our operational or security systems or infrastructure, or those of our third-party vendors or other service providers, including as a result of a cyber-attack;

•costs and effects of litigation, investigations or similar matters to which we may be subject;

•natural disasters, severe weather, acts of god, acts of war or terrorism, domestic civil unrest, geopolitical instability, public health outbreaks (such as coronavirus), other international or domestic calamities, and other events beyond our control, including as a result of in the policies of the current U.S. presidential administration or Congress;

•the impacts of tariffs, sanctions and other trade policies of the United States and its global trading counterparts and the resulting impact on the Company and its customers;

S-2

•compliance with governmental and regulatory requirements, including the Dodd-Frank Act Wall Street Reform and Consumer Protection Act, Economic Growth, Regulatory Relief, and Consumer Protection Act of 2018, and others relating to banking, consumer protection, securities and tax matters;

•changes in accounting principles and standards;

•changes in the laws, rules, regulations, interpretations or policies that apply to the Company's business and operations, and any additional regulations, or repeals that may be forthcoming as a result thereof, which could cause the Company to incur additional costs and adversely affect the Company's business environment, operations, and financial results; and

•our ability to navigate the uncertain impacts of current and future governmental monetary and fiscal policies, including the current and future policies of the Board of Governors of the Federal Reserve System ("Federal Reserve").

When evaluating forward-looking statements to make decisions about the Company and our common stock, investors should not consider the above list of factors to be a complete list of the risks or uncertainties that we face or that could emerge in the future. All forward-looking statements speak only as of the date of the document in which they are contained. We disclaim any obligation to update these forward-looking statements unless required by law, and we caution you not to place undue reliance on them.

S-3

PROSPECTUS SUPPLEMENT SUMMARY

The following summary should be read together with the information contained in other parts of this prospectus supplement and the accompanying base prospectus. This summary highlights selected information from this prospectus supplement and the accompanying base prospectus to help you understand this offering of our common stock. This summary is not complete and does not contain all of the information that you should consider before deciding whether to invest in our common stock. You should read this prospectus supplement and the accompanying base prospectus, including the documents we incorporate by reference, carefully to understand fully the terms of our common stock as well as the other considerations that are important to you in making a decision about whether to invest in our common stock. You should pay special attention to the "RISK FACTORS" section of this prospectus supplement and the "Risk Factors" described in our Annual Report on Form 10-K for the year ended December 31, 2025 and the other documents incorporated by reference into this prospectus supplement, before you determine whether an investment in our common stock is appropriate for you.

Isabella Bank Corporation

Isabella Bank Corporation is a Michigan corporation and financial holding company incorporated in September 1988. The Company's wholly-owned subsidiaries, Isabella Bank, a Michigan-chartered commercial bank organized and operating since 1903, has 31 offices located throughout Bay, Clare, Gratiot, Isabella, Mecosta, Midland, Montcalm, and Saginaw counties. Both the Company and the Bank are headquartered in Mount Pleasant, Michigan.

The Bank is a community bank with a focus on providing high quality, personalized service at a fair price. The Bank offers a broad array of banking and wealth management services to businesses, institutions, individuals, and their families. Lending activities include loans for commercial and agricultural operations and real estate purposes, residential real estate loans, and consumer loans. The Bank limits lending activities primarily to its local markets and purchased loans from the secondary market are minimal. The Bank does not make loans to fund leveraged buyouts, has no foreign corporate or government loans, and has limited holdings of corporate debt securities. Our general lending philosophy is to limit concentrations to individuals and business segments. Deposit services offered include checking accounts, savings accounts, certificates of deposit, direct deposits, cash management services, mobile and internet banking, and ATMs. We also offer full-service investment management, trust, and estate services.

At March 31, 2026, we had, on a consolidated basis, total assets of approximately $2.3 billion, total gross loans of approximately $1.6 billion, total deposits of approximately $1.9 billion, and total shareholders' equity of approximately $234.0 million.

The Company's and the Bank's principal offices are located at 401 North Main Street, Mount Pleasant, Michigan 48858 and our telephone number is (989) 772-9471.

Recent Developments

Pending Merger with Grand River Commerce, Inc.

On June 11, 2026, we entered into an Agreement and Plan of Merger (the "merger agreement") with Grand River Commerce, Inc. ("Grand River"), the parent holding company of Grand River Bank. The merger agreement provides that the following transactions will occur: (i) A newly-formed subsidiary of Purchaser will merge with and into Grand River, with Grand River continuing as the surviving corporation in the merger, (ii) subsequently, Grand River will merge with and into the Company, with the Company continuing as the surviving corporation in the merger, and (iii) subsequently, Grand River Bank will merge with and into Isabella Bank, with Isabella Bank continuing as the surviving bank in the bank merger (the "Grand River acquisition").

The Grand River acquisition is expected to close in the fourth quarter of 2026, subject to the satisfaction of customary closing conditions, including regulatory approvals and the approval of the shareholders of Grand River. There can be no assurance that the Grand River acquisition will be completed in the anticipated time frame, or at all, or that the anticipated benefits of the Grand River acquisition will be realized. See "Risks Related to the Grand River Acquisition." This offering is not conditioned upon the completion of the Grand River acquisition, nor is the Grand River acquisition conditional upon any shares being sold in this offering.

S-4

For a full description of the material terms of the merger agreement, please see our Current Reports on Form 8-K filed with the SEC on June 12, 2026 and June 15, 2026.

Implications of Being a Smaller Reporting Company

We are currently a "smaller reporting company" as defined in the Exchange Act. We will be a smaller reporting company and may continue to take advantage of the scaled-back disclosures available to smaller reporting companies for so long as (i) the market value of our common stock held by non-affiliates is less than $250.0 million measured on the last business day of our most recently completed second fiscal quarter or (ii) at the time of determination at the end of the fiscal year, (a) our annual revenue is less than $100.0 million during the most recently completed fiscal year for which audited financial statements are then-available and (b) the market value of our common stock held by non-affiliates is less than $700.0 million measured on the last business day of our most recently completed second fiscal quarter. As a smaller reporting company, we are permitted to comply with scaled-back disclosure obligations in our SEC filings compared to other issuers, including with respect to disclosure obligations regarding executive compensation in our periodic reports and proxy statements. We may elect to adopt some or all of the accommodations available to smaller reporting companies. Until we cease to be a smaller reporting company, the scaled-back disclosure in our SEC filings will result in less information about our company being available than for other public companies.

S-5

The Offering

The following is a brief summary of certain terms of this offering. For a more complete description of the terms of the common stock offered hereby, see the "DESCRIPTION OF COMMON STOCK" section of the accompanying base prospectus.



Issuer:			Isabella Bank Corporation, a Michigan corporation and registered financial holding company.

Shares of Common Stock Offered by Us:			Shares of our common stock having an aggregate offering price of up to $30 million.

Manner of Offering:			"At-the-market" offering that may be made from time to time through Piper Sandler, as the Sales Agent. See "PLAN OF DISTRIBUTION" on page S-13 for more information.

Use of Proceeds:			The net proceeds of the offering will be used for general corporate purposes, which may include, without limitation, contribution to the capital of our subsidiary, Isabella Bank, to support its lending activities and growth. Allocations of the net proceeds from this offering for specific purposes have not been made as of the date of this prospectus supplement. The precise amounts and timing of the application of any net proceeds will depend upon our business operations and funding requirements. See "USE OF PROCEEDS" on page S-15 for more information.

Listing:			Our common stock is listed on the Nasdaq Capital Market under the symbol "ISBA."

Risk Factors:			An investment in our common stock involves risks. You should carefully consider the information set forth in the section entitled "RISK FACTORS" beginning on page S-7, as well as other information included or incorporated by reference in this prospectus supplement before deciding whether to invest in our common stock.

Transfer Agent and Registrar:			Isabella Bank Corporation.

S-6

RISK FACTORS

An investment in our securities involves a high degree of risk. Before deciding whether to invest in our common stock, you should carefully read and consider the risks, uncertainties and assumptions discussed below and in Item 1A, "Risk Factors," in our most recent Annual Report on Form 10-K and any updated information described in our subsequent annual reports on Form 10-K, quarterly reports on Form 10-Q and current reports on Form 8-K that we file with the SEC after the date of this prospectus supplement, all of which are incorporated by reference herein and in the accompanying base prospectus. See "INFORMATION INCORPORATED BY REFERENCE" on page S-18 and "WHERE YOU CAN FIND MORE INFORMATION" on page S-19.

Risks Associated with this Offering and our Common Stock

Future sales or issuances of our common stock in the public markets, or the perception of such sales, could depress the trading price of our common stock.

The sale of a substantial number of shares of our common stock in the public markets, or the perception that such sales could occur, could depress the market price of our common stock and impair our ability to raise capital through the sale of additional equity securities. We may sell large quantities of our common stock at any time pursuant to this prospectus supplement or in one or more separate offerings. We cannot predict the effect that future sales of common stock would have on the market price of our common stock. In addition, the Company anticipates that it will issue approximately 839,003 shares of common stock in the Grand River acquisition. The issuance of these new shares of common stock could have the effect of depressing the market price of our common stock, through dilution of earnings per share or otherwise.

Furthermore, former Grand River shareholders and current shareholders of the Company may not wish to continue to invest in the additional operations of the combined company, or for other reasons may wish to dispose of some or all of their interests in the combined company, and as a result may seek to sell their shares of common stock following, or in anticipation of, completion of the Grand River acquisition. These sales (or the perception that these sales may occur), coupled with the increase in the outstanding number of shares of common stock, may affect the market for, and the market price of, common stock in an adverse manner.

If the merger is completed and shareholders of the Company sell substantial amounts of common stock in the public market following the consummation of the Grand River acquisition, the market price of common stock may decrease. These sales might also make it more difficult for the Company to raise capital by selling equity or equity-related securities at a time and price that it otherwise would deem appropriate.

The actual number of shares we will issue under the Equity Distribution Agreement, at any one time or in total, and the amount and timing of any resulting net proceeds, are uncertain.

Subject to certain limitations in the Equity Distribution Agreement and compliance with applicable law, we have the discretion to deliver a placement notice to the Sales Agent at any time throughout the term of the Equity Distribution Agreement. The number of shares that are sold by the Sales Agent after delivering a placement notice will fluctuate based on the market price of our common stock during the sales period and limits we set with the Sales Agent. Because the timing of any sales is not known at this time and the price per share of each share sold will fluctuate based on the market price of our common stock during any sales periods, it is not possible at this stage to predict the number of shares that will be ultimately issued or the amount or timing of any net proceeds we may receive.

We will have broad discretion in the use of any net proceeds from this offering and may not use them effectively.

Our senior management will have broad discretion in the application of any net proceeds from this offering. Because of the number and variability of factors that will determine the amount and use of the net proceeds, their ultimate use may vary substantially from their currently intended use. Our senior management might not apply our net proceeds in ways that ultimately increase the value of your investment. While we expect to use the net proceeds from this offering as described under "USE OF PROCEEDS" on page S-15, we are not obligated to do so. The failure by our management to apply these funds effectively could harm our business. If we do not invest or apply the net proceeds in ways that enhance shareholder value, we may fail to achieve expected financial results, which could adversely affect our business, financial condition and results of operations, and cause the price of our common stock to decline.

S-7

The common stock offered hereby will be sold in "at the market" offerings, and investors who buy shares at different times will likely pay different prices.

Investors who purchase shares in this offering at different times will likely pay different prices, and so may experience different outcomes in their investment results. We will have discretion, subject to market demand, to vary the timing, prices, and numbers of shares sold, and there is no minimum or maximum sales price. Investors may experience a decline in the value of their shares as a result of share sales made at prices lower than the prices they paid.

You may experience future dilution as a result of future equity offerings.

To raise additional capital, we may in the future offer additional shares of common stock or other securities convertible into or exchangeable for common stock. Because our decision to issue securities in any future offering will depend on market conditions and other factors, some of which are beyond our control, we cannot predict or estimate the amount, timing or nature of any future offerings, or the prices at which such offerings may be effected. We may sell common stock or other securities in any other offering at a price per share that is less than the price per share paid by investors in this offering, and investors purchasing shares or other securities in the future could have voting, liquidation or other rights superior to existing shareholders. The price per share at which we sell additional common stock, or securities convertible or exchangeable into common stock, in future transactions may be higher or lower than the price per share paid by investors in this offering. Additional equity offerings may dilute the holdings of our existing shareholders or reduce the market price of our common stock, or both. Holders of our common stock are not entitled to preemptive rights or other protections against dilution.

The market price of our common stock may fluctuate significantly, and there is a limited trading volume in our common stock. This may make it difficult for you to resell shares of common stock owned by you at times or at prices you find attractive.

The market price of our common stock on the Nasdaq constantly changes. We expect that the market price of our common stock will continue to fluctuate and there can be no assurance about the market price for our common stock in the future. In addition, there is limited trading volume in our common stock, and we do not expect that this offering will result in a substantially more liquid market for our common stock.

Stock price volatility and the limited trading volume in our stock may make it difficult for you to resell your common stock when you want and at prices you find attractive. Our stock price may fluctuate significantly as a result of a variety of factors, many of which are beyond our control, including those described above under "CAUTIONARY NOTE REGARDING FORWARD-LOOKING STATEMENTS" and in our annual and other periodic reports filed with the SEC and incorporated by reference herein.

General market fluctuations, industry factors and general economic and political conditions and events, such as economic slowdowns or recessions, geopolitical tensions, interest rate changes, credit loss trends and federal tariff and trade policy, could also cause our stock price to decrease regardless of our operating results due to volatility and disruption of capital and credit markets.

Common stock is equity and is subordinate to our existing and future indebtedness and effectively subordinated to all the indebtedness and other non-common equity claims against our subsidiaries.

Shares of our common stock are equity interests in us and do not constitute indebtedness. As such, shares of our common stock will rank junior to all of our indebtedness, including without limitation, our fixed-to-floating rate subordinated notes due 2031, and to other non-equity claims against us and our assets available to satisfy claims against us, including in the event of our liquidation. In the event of our liquidation, lenders and holders of our debt securities would receive distributions of our available assets prior to holders of our common stock. Furthermore, our right to participate in a distribution of assets upon the liquidation or reorganization of Isabella Bank or other subsidiaries would be subject to the prior claims of that subsidiary's creditors, including holders of any preferred stock of that subsidiary.

S-8

We may borrow funds or issue additional debt which would be senior to our common stock as to distributions and in liquidation, and which could negatively affect the value of our common stock.

In the future, we may attempt to increase our capital resources by entering into debt or debt-like financing that is unsecured or secured by all or up to all of our assets, or by issuing additional debt or equity securities, which could include issuances of secured or unsecured commercial paper, medium-term notes, senior notes, subordinated notes, common stock or securities convertible into or exchangeable for common stock. In the event of our liquidation, our lenders and holders of our debt would receive a distribution of our available assets before distributions to the holders of our common stock. Because our decision to incur debt and issue securities in our future offerings will depend on market conditions and other factors beyond our control, we cannot predict or estimate with certainty the amount, timing or nature of our future offerings and debt financings. Further, market conditions could require us to accept less favorable terms for the issuance of our securities in the future. In addition, the borrowing of funds or issuance of debt would increase our leverage and decrease our liquidity, and the issuance of additional equity securities would dilute the interests of our existing shareholders.

An investment in our common stock is not a deposit insured by the Federal Deposit Insurance Corporation ("FDIC").

Our shares of common stock are not savings accounts, deposits or other obligations of any of Isabella Bank, and are not insured or guaranteed by the FDIC or any other governmental agency or instrumentality. Your investment in the Company will be subject to investment risk and you may experience a complete loss of your investment.

You may not receive dividends on the common stock.

Holders of our common stock are only entitled to receive such dividends as our board of directors may declare out of funds legally available for such payments. Even if we have earnings in an amount sufficient to pay cash dividends, our board of directors may decide to retain earnings for the purpose of funding growth. We cannot assure you that cash dividends on our common stock will ever be paid. You should not purchase shares of common stock offered hereby if you need or desire dividend income from this investment.

Furthermore, as a financial holding company, our ability to pay dividends is subject to the guidelines of the Federal Reserve regarding capital adequacy and dividends. It is the policy of the Federal Reserve that bank and financial holding companies should generally pay dividends on capital stock only out of earnings, and only if prospective earnings retention is consistent with the organization's expected future needs, asset quality and financial condition. Dividends also may be limited as a result of safety and soundness considerations.

Our liquidity is dependent on dividends from the Bank.

The Company is a legal entity separate and distinct from the Bank, whose primary source of funds consists of dividends from the Bank. Various federal and state laws and regulations limit the amount of dividends that the Bank may pay to the Company. Also, the right of the Company and consequently the right of shareholders of the Company, to participate in any distribution of the assets or earnings of its subsidiaries, through the payment of such dividends or otherwise, is subject to the prior claims of creditors of the subsidiaries, including, with respect to Isabella Bank, depositors of Isabella Bank, except to the extent that certain claims of the Company in a creditor capacity may be recognized. In the event the Bank is unable to pay dividends to us, we may not be able to service any debt we may incur, which could have a material adverse effect on our business, financial condition, results of operations and growth prospects.

The FDIC also has the authority to use its enforcement powers to prohibit Isabella Bank from paying dividends if, in its opinion, the payment of dividends would constitute an unsafe or unsound practice. Federal law also prohibits the payment of dividends by Isabella Bank that will result in Isabella Bank failing to meet its applicable capital requirements on a pro forma basis.

Ownership of our common stock may require regulatory approval or result in adverse regulatory consequences.

We are a financial holding company regulated by the Federal Reserve. Any "company", as defined in the Bank Holding Company Act of 1956, as amended, or the "BHC Act," owning 25% or more of a class of our outstanding shares of voting stock, or a lesser percentage if such holder otherwise exercises a "controlling influence" over us, may be subject to regulation as a "bank holding company" in accordance with the BHC Act. A holder or group of holders acting in concert may also be deemed to control us if they own one-third or more of our total equity, both voting and

S-9

non-voting, aggregating all shares held by the investor across all classes of stock. A company determined to control us under the BHC Act will be subject to ongoing regulation and supervision.

In addition, no person, including individuals or groups of individuals acting in concert may acquire "control" of us under the Change in Bank Control Act, or the "CBC Act," without providing prior notice to the Federal Reserve and receiving a nonobjection from the Federal Reserve.

Further, acquisitions of our voting stock above certain thresholds may be subject to prior regulatory notice or approval under the BHC Act or the CBC Act. For example, any bank holding company or foreign bank that is subject to the BHC Act may need approval to acquire or retain more than 5% of the then outstanding shares in a class of voting stock, and any holder (or group of holders acting in concert) may need regulatory approval to acquire or retain 10% or more of the shares in a class of voting stock.

Under either the BHC Act or the CBC Act, a regulatory determination of "control" of a depository institution or holding company is based on all of the relevant facts and circumstances. Potential investors are advised to consult with their legal counsel regarding the applicable regulations and requirements.

Risks Related to the Grand River Acquisition

The Grand River acquisition may not be completed and the merger agreement may be terminated in accordance with its terms. Failure to complete the Grand River acquisition could negatively impact the price of shares of our common stock, as well as our future business and financial results.

We expect the Grand River acquisition to close during the fourth quarter of 2026, but the acquisition is subject to a number of closing conditions. Satisfaction of many of these conditions is beyond our control. If these conditions are not satisfied or waived, the Grand River acquisition will not be completed. Certain of the conditions that remain to be satisfied include, but are not limited to:

•the continued accuracy of the representations and warranties made by the parties in the merger agreement;

•the performance by each party of its respective obligations under the merger agreement;

•the approval by Grand River shareholders of the merger agreement and the Grand River acquisition;

•the receipt of required regulatory approvals, including the approval of the Federal Reserve and the Michigan Department of Insurance and Financial Services; and

•the absence of any injunction, order or decree restraining, enjoining or otherwise prohibiting the Grand River acquisition.

As a result, the Grand River acquisition may not close as scheduled, or at all. In addition, either the Company or Grand River may terminate the merger agreement under certain circumstances. Without realizing any of the benefits of having completed the transactions, we will be subject to a number of risks, including the following:

•we may be required to pay our costs relating to the transactions, which are substantial, such as legal, accounting, and financial advisory fees, whether or not the transactions are completed;

•time and resources committed by management to matters relating to the transactions could otherwise have been devoted to pursuing other beneficial opportunities;

•we may experience negative reactions from financial markets, including negative impacts on the prices of our common stock, including to the extent that the current market price reflects a market assumption that the Grand River acquisition will be completed;

•we may experience negative reactions from employees, customers or vendors;

•Grand River may owe a termination fee of $2,180,000; and

•since the merger agreement restricts the conduct of our business prior to completion of the Grand River acquisition, we may not have been able to take certain actions during the pendency of the merger that would have benefited us as an independent company and the opportunity to take such actions may no longer be available.

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If the transactions contemplated by the merger agreement are not completed for any reason, our ongoing business, financial condition and financial results may be adversely affected. In addition, any delay in completing the merger may significantly reduce the synergies and other benefits that we expect that the combined company may achieve if the merger is completed within the expected timeframe.

Regulatory approvals may not be received, may take longer than expected or may impose conditions that we do not anticipate or cannot be met.

Before the Grand River acquisition may be completed, various approvals or waivers must be obtained from bank regulatory authorities, including the Federal Reserve and the Michigan Department of Insurance and Financial Services. These regulators may impose conditions on the completion of, or require changes to the terms of, the Grand River acquisition. Such conditions or changes and the process of obtaining regulatory approvals or waivers could have the effect of delaying completion of the Grand River acquisition or of imposing additional costs or limitations on us following the completion of the Grand River acquisition. The regulatory approvals or waivers may not be received at all, may not be received in a timely fashion or may contain conditions on the completion of the Grand River acquisition that are burdensome, not anticipated, or cannot be met. If the completion of the Grand River acquisition is delayed, including by a delay in receipt of necessary governmental approvals or waivers, the business, financial condition, and results of operations of the Company and Grand River may also be materially adversely affected.

We may be unsuccessful in integrating the operations of the businesses we have acquired or expect to acquire in the future, including Grand River.

From time to time, we evaluate and acquire businesses that we believe complement our existing business. The acquisition component of our growth strategy depends on the successful integration of these acquisitions. We face numerous risks and challenges to the successful integration of acquired businesses, including the following:

•the potential for unexpected costs, delays and challenges that may arise in integrating acquisitions into our existing business;

•limitations on our ability to realize the expected cost savings and synergies from an acquisition;

•challenges related to integrating acquired operations, including our ability to retain key employees and maintain relationships with significant customers and depositors;

•challenges related to the integration of businesses that operate in new geographic areas, including difficulties in identifying and gaining access to customers in new markets; and

•the discovery of previously unknown liabilities following an acquisition associated with the acquired business.

If we are unable to successfully integrate the businesses we acquire, our business, financial condition and results of operations may be materially adversely affected.

The Grand River acquisition could result in unexpected disruptions on the combined business.

In response to the announcement of the Grand River acquisition, Grand River's customers may cease or reduce their business with Grand River, which could negatively affect our combined business operations. Similarly, current or prospective employees of the Company or of Grand River may experience uncertainty about their future roles with the combined entity. This may adversely affect our ability to attract and retain key management, banking and other personnel. In addition, the diversion of the attention of our respective management teams away from day-to-day operations during the negotiation and pendency of the Grand River acquisition could have an adverse effect on the financial condition and operating results of either us or Grand River.

We or Grand River or both may be subject to claims and litigation pertaining to the merger that could prevent or delay the completion of the merger.

Any lawsuits filed in connection with the proposed merger could prevent or delay completion of the merger and result in substantial costs to the Company and Grand River, including any costs associated with indemnification. The defense or settlement of any lawsuit or claim that may be filed seeking remedies against the Company, our board of directors

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or Grand River or its board of directors in connection with the merger that remains unresolved at the effective time of the merger may adversely affect the Company's business, financial condition, results of operations and cash flows.

We may fail to realize some or all of the anticipated benefits of the Grand River acquisition.

The success of the Grand River acquisition will depend, in part, on our ability to realize the anticipated benefits and cost savings from combining our business with Grand River's business. However, to realize these anticipated benefits and cost savings, we must successfully combine both businesses. If we are not able to achieve these objectives, the anticipated benefits and cost savings of the Grand River acquisition may not be realized fully, or at all, or may take longer to realize than we expect.

We will incur significant transaction and merger-related integration costs in connection with the Grand River acquisition.

We expect to incur significant costs associated with completing the Grand River acquisition and integrating Grand River's operations into our operations and are continuing to assess the impact of these costs. Although we believe that the elimination of duplicate costs, as well as the realization of other efficiencies related to the integration of Grand River's business with the Company's business, will offset incremental transaction and merger-related costs over time, this net benefit may not be achieved in the near term, or at all.

The Grand River acquisition may be completed on different terms from those contained in the merger agreement.

Prior to the completion of the Grand River acquisition, the Company and Grand River may, by mutual agreement, amend or alter the terms of the merger agreement, including with respect to, among other things, the merger consideration payable by us to Grand River's shareholders or any covenants or agreements with respect to the parties' respective operations during the pendency thereof. Any such amendments or alterations may have negative consequences to the Company.

We may not be able to implement aspects of our growth strategy or new bank facilities may not be profitable.

We believe the Company's growth strategy contemplates the future expansion of our business and operations both organically and through acquisitions. Implementing these aspects of our growth strategy depends, in part, on our ability to successfully identify acquisition opportunities and strategic partners that will complement our operating philosophy and to successfully integrate their operations with ours, as well as to generate loans and deposits within acceptable risk and expense tolerances. the Company may not be able to identify suitable opportunities for further growth and expansion or, if we do, we may not be able to successfully integrate these new operations into our business.

As consolidation of the financial services industry continues, the competition for suitable acquisition candidates may increase. the Company will compete with other financial services companies for acquisition opportunities, and many of these competitors have greater financial resources than the Company does and may be able to pay more for an acquisition than we are willing or able to pay.

We can offer no assurance that we will have opportunities to acquire other financial institutions, or that we will complete the Grand River acquisition, or acquire or establish any new branches or mortgage offices, or that we will be able to negotiate, finance and complete any opportunities available to us. Additionally, the Company may not be able to organically expand into new markets that are profitable for its business. The costs to start up new bank branches and loan production offices in new markets and the additional costs to operate these facilities would increase the Company's non-interest expense and may decrease earnings. It may be difficult to adequately and profitably manage growth through the establishment of bank branches and loan production offices in new markets. In addition, we can provide no assurance that our expansion into any such new markets will successfully attract enough new business to offset the expenses of their operation. If we are not able to do so, the Company's earnings and stock price may be negatively impacted.

Risks Relating to Our Business

For a discussion of risks relating to our business and operations, including risks related to credit and interest rates, the economy, and our operations and strategy, see the section entitled "Risk Factors" in our Annual Report on Form 10-K

S-12

for the year ended December 31, 2025, which is incorporated into this prospectus supplement by reference, as the same may be updated from time to time prior to the completion of this offering by our future filings under the Exchange Act.

The risks and uncertainties we have described above and those incorporated by reference herein are not the only ones we face. Additional risks and uncertainties not presently known to us or that we currently deem immaterial may also affect our business and operations. The occurrence of any of these known or unknown risks, or changes in management's assumptions in evaluating the risks we face, might cause you to lose all or part of your investment in our common stock.

S-13

PLAN OF DISTRIBUTION

We have entered into the Equity Distribution Agreement with Piper Sandler, under which we may offer and sell up to $30,000,000 of our shares of common stock from time to time through Piper Sandler acting as Sales Agent. Sales of our shares of common stock, if any, under this prospectus supplement and the accompanying base prospectus will be made by the Sales Agent by any method that is deemed to be an "at the market offering" as defined in Rule 415(a)(4) under the Securities Act. In addition, with our prior consent and subject to the terms we may establish, the Agent may also sell the common stock by any other method permitted by law, including privately negotiated transactions.

Each time we wish to issue and sell our shares of common stock under the Equity Distribution Agreement, we will notify the Sales Agent of the number of shares to be issued, the time period during which such sales are requested to be made, any limitation on the number of shares to be sold in any one day and any minimum price below which sales may not be made. Once we have so instructed Piper Sandler, unless Piper Sandler declines to accept the terms of such notice, Piper Sandler has agreed to use its commercially reasonable efforts consistent with its normal trading and sales practices to sell such shares up to the amount specified on such terms. The obligations of the Sales Agent under the Equity Distribution Agreement to sell our shares of common stock are subject to a number of conditions that we must meet.

The settlement of sales of shares between us and the Sales Agent generally will occur on the first full business day following the date on which the sale was made. Sales of our shares of common stock in the offering generally will be settled through the facilities of The Depository Trust Company or by such other means as we and the Sales Agent may agree upon. There is no arrangement for funds to be received in an escrow, trust or similar arrangement.

For its services as Sales Agent, we will pay Piper Sandler an amount up to 3.0% of the aggregate gross sales proceeds from each sale of shares our common stock sold pursuant to the Equity Distribution Agreement. Because there is no minimum offering amount required as a condition to close this offering, the actual total public offering amount, commissions and proceeds to us, if any, are not determinable at this time. In addition, we have agreed to reimburse Piper Sandler for the reasonable and documented fees and disbursements of its legal counsel: (i) payable upon execution of the Equity Distribution Agreement, in an amount not to exceed $100,000 in connection with the Equity Distribution Agreement, the Registration Statement, this prospectus supplement and accompanying base prospectus, and (ii) thereafter, $15,000 for each quarter beginning with the third quarter of 2026. In accordance with the Financial Industry Regulatory Authority, Inc. Rule 5110 these reimbursed fees and expenses are deemed sales compensation in connection with this offering. We estimate that the total expenses for the offering, excluding any commissions or expense reimbursement payable to Piper Sandler under the terms of the Equity Distribution Agreement, will be approximately $250,000.00. The remaining sale proceeds, after deducting any other transaction fees, will equal our net proceeds from the sale of the shares.

Piper Sandler will provide written confirmation to us no later than the open of the Nasdaq Capital Market on the trading day immediately following each day on which our shares of common stock are sold under the Equity Distribution Agreement. Each confirmation will include the number of shares sold on that day, the volume-weighted average price of the shares sold and the proceeds to us of such shares, net of the sales commission.

In connection with the sale of our shares of common stock on our behalf, Piper Sandler will be deemed to be an "underwriter" within the meaning of the Securities Act, and the compensation of Piper Sandler will be deemed to be underwriting commissions or discounts. We have agreed to indemnify Piper Sandler against certain civil liabilities, including liabilities under the Securities Act. We have also agreed to contribute to payments Piper Sandler may be required to make in respect of such liabilities.

The offering of our shares of common stock pursuant to the Equity Distribution Agreement will terminate upon the earlier of (i) the sale of all shares of common stock subject to the Equity Distribution Agreement and (ii) the termination of the Equity Distribution Agreement as permitted therein. We and Piper Sandler may each terminate the sales agreement at any time upon specified prior notice.

This summary of the material provisions of the Equity Distribution Agreement does not purport to be a complete statement of its terms and conditions. A copy of the Equity Distribution Agreement is filed as an exhibit to the Company's current report on Form 8-K dated June 16, 2026 filed under the Exchange Act and incorporated by reference in this prospectus supplement. (See "INFORMATION INCORPORATED BY REFERENCE" on page S-18.)

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Piper Sandler and its affiliates have provided, and may in the future provide, various investment banking, commercial banking, financial advisory and other financial services for us, for which services they have received, and may in the future receive, customary fees. In the course of its business, Piper Sandler may actively trade our securities for its own account or for the accounts of customers, and, accordingly, Piper Sandler may at any time hold long or short positions in such securities. However, neither Piper Sandler nor its affiliates will engage in transactions in our common stock that are intended to stabilize or maintain the market price of our common stock.

A prospectus supplement and the accompanying base prospectus in electronic format may be made available on a website maintained by Piper Sandler, and Piper Sandler may distribute the prospectus supplement and the accompanying base prospectus electronically.

S-15

USE OF PROCEEDS

We are not guaranteed to receive any particular amount of proceeds from this offering. The amount of the proceeds we receive from this offering will depend upon the number of shares of our common stock sold and the market price at which they are sold.

We expect to use the net proceeds from the sale of the securities for our general corporate purposes, which may include, among other purposes, contribution to the capital of our subsidiary, Isabella Bank, to support its lending, investing and other banking activities; to repay indebtedness; and to support or fund acquisitions and other strategic initiatives and activities permissible for financial holding companies.

S-16

EXPERTS

The consolidated financial statements of Isabella Bank Corporation for the year ended December 31, 2025, and the effectiveness of internal control over financial reporting as of December 31, 2025, incorporated by reference herein from the Isabella Bank Corporation Annual Report on Form 10-K for the year ended December 31, 2025, have been audited by Plante Moran PLLC, an independent registered public accounting firm, as stated in their reports thereon incorporated herein by reference, and have been incorporated in this prospectus and registration statement in reliance upon such reports and upon the authority of such firm as experts in accounting and auditing.

The consolidated financial statements of Isabella Bank Corporation as of December 31, 2024 and 2023, and for each of the years in the two-year period ended December 31, 2024, and the effectiveness of internal control over financial reporting as of December 31, 2024, incorporated by reference herein from the Isabella Bank Corporation Annual Report on Form 10-K for the year ended December 31, 2024, have been audited by Rehmann Robson LLC, an independent registered public accounting firm, as stated in their reports thereon incorporated herein by reference, and have been incorporated in this prospectus and registration statement in reliance upon such reports and upon the authority of such firm as experts in accounting and auditing.

S-17

LEGAL MATTERS

The validity of the shares of the Company's common stock offered by this prospectus will be passed upon for the Company by Foster Swift Collins & Smith P.C., Lansing, Michigan. Certain legal matters will be passed upon for the Company by Hunton Andrews Kurth LLP, Dallas, Texas. Covington & Burling LLP, Boston, Massachusetts, is acting as counsel to the Sales Agent in connection with this offering.

S-18

INFORMATION INCORPORATED BY REFERENCE

The SEC allows the Company to incorporate by reference into this prospectus supplement and accompanying base prospectus information and reports that we file with it. This means that we may disclose important information to you by referring you to another document filed separately with the Commission. It also means that information and reports that we subsequently file with the Commission will automatically update and supersede information contained in our earlier-filed reports and in this prospectus supplement and the accompanying base prospectus. This prospectus supplement incorporates by reference the documents which are listed below that the Company has previously filed with the Commission. These documents contain important information about the Company, including our business, financial condition and results of operations.

The following documents previously filed by us with the Commission under the Exchange Act are incorporated by reference into this Registration Statement:

•Our Annual Report on Form 10-K for the year ended December 31, 2025, filed with the SEC on March 13, 2026;

•our Quarterly Report on Form 10-Q for the quarter ended March 31, 2026, filed with the SEC on May 5, 2026;

•our Definitive Proxy Statement on Schedule 14A, filed with the SEC on March 23, 2026 (solely to the extent incorporated by reference into Part III of our Annual Report on Form 10-K for the year ended December 31, 2025);

•our Current Reports on Form 8-K filed on January 5, 2026, February 26, 2026, March 25, 2026; May 7, 2026; May 28, 2026; June 12, 2026; and June 15, 2026 (other than any information contained therein that was furnished but not filed under Item 7.01); and

•the description of our common stock, no par value, contained under the caption "Description of Registrant's Securities to be Registered" set forth in the Registration Statement on Form 8-A12b (File No. 001-42639), filed pursuant to Section 12 of the Exchange Act, on May 8, 2025, including any amendment or report filed for the purpose of updating such description.

We also incorporate by reference into this prospectus supplement and accompanying base prospectus all documents we file with the SEC under Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act on or after the date of filing of our Registration Statement containing this prospectus and prior to the termination of the offering and the filing of a post-effective amendment to our Registration Statement which indicates that all securities offered have been sold or which deregisters all securities then remaining unsold hereunder, except that we are not incorporating by reference any information furnished (but not filed) under Item 2.02 or Item 7.01 of any Current Report on Form 8-K, including the related exhibits under Item 9.01.

Any statement contained in a document incorporated by reference in this prospectus supplement shall be deemed to be modified or superseded for purposes of this prospectus to the extent that a statement contained in this prospectus, or in any other document filed later that is also incorporated in this prospectus by reference, modifies or supersedes the statement. Any statement so modified or superseded shall not be deemed to constitute a part of this prospectus supplement or accompanying base prospectus, except as so modified or superseded. The information relating to us contained in this prospectus supplement and accompanying base prospectus should be read together with the information contained in any documents incorporated by reference herein.

We will provide to you without charge a copy of any or all of the documents incorporated by reference into this prospectus supplement or the accompanying base prospectus, other than exhibits to such documents, unless such exhibits are specifically incorporated by reference into this prospectus supplement or accompanying base prospectus. Requests should be made in writing or by telephone, directed to:

S-19

401 N. Main Street

Mt. Pleasant, Michigan 48858

(989) 772-9471

Attention: Corporate Secretary

S-20

WHERE YOU CAN FIND MORE INFORMATION

We file annual, quarterly and current reports, proxy statements and other information with the SEC. Our filings with the Commission are available to the public through the SEC's website, at http://www.sec.gov.

We also make available free of charge on our website, at http://www.ir.isabellabank.com, the documents that we file with the SEC as soon as reasonably practicable after they are filed electronically with the Commission. Information contained on, or that can be accessed through, our website is not incorporated by reference into this prospectus supplement or the accompanying base prospectus, and you should not consider such information as part of this prospectus supplement or accompanying base prospectus.

This prospectus is part of a Registration Statement that we filed with the SEC and does not contain all of the information in the Registration Statement. The full Registration Statement, including exhibits to the Registration Statement, provides additional information about us and the securities offered under this prospectus supplement and accompanying base prospectus and may be obtained from the SEC or us, as noted above.

S-21

$75,000,000

Debt Securities

Common Stock

Subscription Rights

Warrants

Units

Isabella Bank Corporation ("we," "us," "our" or "the Company") may offer and sell, at any time and from time to time, in one or more offerings, together or separately, any combination of the securities described in this prospectus. The securities we may offer may be convertible into or exchangeable for other securities. The maximum aggregate initial public offering price of the securities offered through this prospectus is $75,000,000, or the foreign currency equivalent thereof.

This prospectus describes some of the general terms that may apply to these securities and the general manner in which they may be offered. Each time that we offer and sell securities using this prospectus, we will provide a supplement to this prospectus that contains specific information about the securities and their terms and the manner in which we will offer them for sale. The prospectus supplement may also add or update information contained in this prospectus. This prospectus may not be used to sell securities unless accompanied by the applicable prospectus supplement. You should carefully read this prospectus and any supplement to this prospectus, as well as any documents we have incorporated into this prospectus by reference, before you invest in any of these securities. References herein to "prospectus supplement" are deemed to refer to any pricing supplement or free writing prospectus describing the specific pricing or other terms of the applicable offering that we prepare and distribute.

We may offer and sell these securities to or through one or more underwriters, dealers and agents, or directly to purchasers, on a continuous or delayed basis. If an offering of securities involves any underwriters, dealers or agents, we will provide the names of any such underwriters, dealers or agents used in connection with the sale of any of these securities, as well as any fees, commissions or discounts we may pay to such underwriters, dealers or agents in connection with the sale of these securities, in the applicable prospectus supplement. The prospectus supplement for each offering of securities will describe in detail the plan of distribution for that offering. See the sections of this prospectus entitled "About This Prospectus" and "Plan of Distribution" for more information. Net proceeds from the sale of securities will be set forth in the applicable prospectus supplement.

Our common stock is listed for trading on the Nasdaq Capital Market under the symbol "ISBA." We have not yet determined whether any of the other securities that may be offered by this prospectus will be listed on any exchange. If we decide to do so, a prospectus supplement relating to such securities will identify the exchange or market on which they will be listed.

Our principal executive office is located at 401 N. Main Street, Mt. Pleasant, Michigan 48858, and our telephone number at that address is (989) 772-9471.

These securities are not savings accounts, deposits or other obligations of any of our bank or non-bank subsidiaries and are not insured by the Federal Deposit Insurance Corporation or any other governmental agency, nor are they obligations of, or guaranteed by, a bank or savings association.

Neither the U.S. Securities and Exchange Commission, nor any state securities commission, the Federal Deposit Insurance Corporation, the Board of Governors of the Federal Reserve System nor any other regulatory body has approved or disapproved of these securities or determined if this prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

Investing in our securities involves risks. See "Risk Factors" beginning on page 8 of this prospectus and contained in our most recent Annual Report on Form 10-K and Quarterly Reports on Form 10-Q, which are incorporated herein by reference, as well as the risk factors included in, or incorporated by reference into, the applicable prospectus supplement, our periodic reports and other information that we file with the U.S. Securities and Exchange Commission.

You should carefully read this prospectus together with the documents we incorporate by reference and the prospectus supplement before you invest in our securities.

This prospectus is not an offer to sell any securities other than the securities offered hereby. This prospectus is not an offer to sell securities in any jurisdictions or in any circumstances in which such an offer is unlawful.

The date of this prospectus is April 23, 2026.

TABLE OF CONTENTS

Page


ABOUT THIS PROSPECTUS			1
INCORPORATION OF CERTAIN INFORMATION BY REFERENCE			2
WHERE YOU CAN FIND MORE INFORMATION			3
SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS			4
ABOUT ISABELLA BANK CORPORATION			6
RISK FACTORS			7
USE OF PROCEEDS			8
THE SECURITIES WE MAY OFFER			9
DESCRIPTION OF DEBT SECURITIES			10
DESCRIPTION OF COMMON STOCK			23
DESCRIPTION OF SUBSCRIPTION RIGHTS			26
DESCRIPTION OF WARRANTS			28
DESCRIPTION OF UNITS			29
PLAN OF DISTRIBUTION			30
LEGAL MATTERS			33
EXPERTS			34

ABOUT THIS PROSPECTUS

This prospectus is part of a registration statement on Form S-3 that we filed with the U.S. Securities and Exchange Commission (the "SEC") using a "shelf" registration process. Under this shelf registration statement, we may offer and sell, from time to time and in one or more offerings, on a continuous or delayed basis, any combination of the securities described in this prospectus for an aggregate offering price of up to $75,000,000, or the foreign currency equivalent thereof.

This prospectus provides you with a general description of the securities we may offer. Each time we sell securities, we will provide a prospectus supplement containing specific information about the terms of that offering. The prospectus supplement may include a discussion of any risk factors or other special considerations that apply to those securities. The prospectus supplement may also add, update or change the information contained or incorporated by reference in this prospectus. If there is any inconsistency between the information in this prospectus (including the information incorporated by reference herein) and information in any prospectus supplement, you should rely on the information in the applicable prospectus supplement. You should carefully read both this prospectus and the applicable prospectus supplement together with additional information described under the heading "Where You Can Find More Information" or incorporated herein by reference as described under the heading "Incorporation of Certain Information by Reference."

You should rely only on the information contained in or incorporated by reference in this prospectus and in the applicable prospectus supplement when deciding whether to invest. We have not authorized anyone to give oral or written information about this offering, the Company, or the securities offered hereby that is different from the information included or incorporated by reference in this prospectus. If anyone provides you with different information, you should not rely on it. You should assume that the information contained in this prospectus is accurate only as of the date on the front cover of this prospectus. Our business, financial condition, results of operations and prospects may have changed since that date.

We are not making an offer to sell or soliciting an offer to buy these securities in any jurisdiction in which the offer or solicitation is not authorized or in which the person making the offer or solicitation is not qualified to do so or to anyone to whom it is unlawful to make the offer or solicitation.

Any of the securities described in this prospectus and in a prospectus supplement may be convertible or exchangeable into, or exercisable for, other securities that are described in this prospectus or will be described in a prospectus supplement, and may be issued separately, together or as part of a unit consisting of two or more securities, which may or may not be separate from one another. The securities offered hereby may include new or hybrid securities developed in the future that combine features of any of the securities described in this prospectus.

The registration statement that contains this prospectus, including the exhibits to the registration statement, also contains additional information about us and the securities offered under this prospectus. You can find the registration statement at the SEC's website mentioned under the heading "Where You Can Find More Information."

References in this prospectus to the "Company," "we," "us" and "our" are to Isabella Bank Corporation and its wholly-owned bank subsidiary, Isabella Bank, unless the context otherwise requires. References in this prospectus to the "Bank" are to Isabella Bank. In this prospectus, we sometimes refer to the debt securities, common stock, warrants, units and subscription rights we may offer as "offered securities."

INCORPORATION OF CERTAIN INFORMATION BY REFERENCE

The SEC allows us to incorporate by reference the information that we file with it into this prospectus, which means that we can disclose important information to you by referring you to other documents. This prospectus incorporates by reference the documents listed below that we have previously filed with the SEC (SEC File Nos. 001-42639 and 000-18415):xx

•our Annual Report on Form 10-K for the year ended December 31, 2025, filed with the SEC on March 13, 2026;

•our Current Reports on Form 8-K filed on January 5, 2026, February 26, 2026, and March 25, 2026; and

We incorporate by reference any additional documents that we may file with the SEC under Sections 13(a), 13(c), 14 or 15(d) of the Securities Exchange Act of 1934, as amended (the "Exchange Act") (other than those furnished pursuant to Item 2.02 or Item 7.01 of Form 8-K or other information "furnished" to the SEC), from the date of the registration statement of which this prospectus is a part until the termination of the offering of the securities offered hereby. These documents may include annual, quarterly and current reports, as well as proxy statements. Any material that we later file with the SEC will automatically update and supersede, where appropriate, the information previously filed with the SEC. These documents are available to you without charge. See "Where You Can Find More Information."

Any statement contained in a document incorporated or deemed to be incorporated herein by reference shall be deemed to be modified or superseded to the extent that a statement contained herein or in any other subsequently filed document that also is or is deemed to be incorporated herein by reference modifies or supersedes such statement in such document.

WHERE YOU CAN FIND MORE INFORMATION

We are subject to the information requirements of the Exchange Act and file with the SEC annual, quarterly and current reports, proxy statements, information statements and other information. Our SEC filings are available to the public at the SEC's web site at www.sec.gov or on our Internet web site, https://www.ir.isabellabank.com. The reference to our web site is not intended to be an active link and the information on our web site is not, and you must not consider the information to be, a part of this prospectus and is not incorporated by reference herein.

We have filed with the SEC a registration statement on Form S-3 under the Securities Act of 1933, as amended (the "Securities Act"), relating to the securities covered by this prospectus. The registration statement, including the attached exhibits and schedules, contains additional relevant information about us and the securities. This prospectus does not contain all of the information set forth in the registration statement. Whenever a reference is made in this prospectus to a contract or other document, the reference is only a summary and you should refer to the exhibits that form a part of the registration statement for a copy of the contract or other document. You can get a copy of the registration statement and the documents referred to above under "Incorporation of Certain Information by Reference" from the SEC web site or our web site listed above. You can also obtain these documents from us, without charge (other than exhibits, unless the exhibits are specifically incorporated by reference), upon written or oral request at the following address and telephone number:

401 N. Main Street

Mt. Pleasant, Michigan 48858

(989) 772-9471

Attention: Corporate Secretary

SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS

This prospectus and the documents incorporated herein by reference contain statements that are "forward-looking statements" within the meaning of Section 27A of the Exchange Act and Section 21E of the Exchange Act. These forward-looking statements reflect our current views with respect to, among other things, future events and our financial performance. These statements are often, but not always, made through the use of words or phrases such as "may," "might," "should," "could," "predict," "potential," "believe," "expect," "continue," "will," "anticipate," "seek," "estimate," "intend," "plan," "strive," "projection," "goal," "target," "outlook," "aim," "would," "annualized" and "outlook," or the negative version of those words or other comparable words or phrases of a future or forward-looking nature. These forward-looking statements are not historical facts, and are based on current expectations, estimates and projections about our industry, management's beliefs and certain assumptions made by management, many of which, by their nature, are inherently uncertain and beyond our control. Accordingly, we caution you that any such forward-looking statements are not guarantees of future performance and are subject to risks, assumptions, estimates and uncertainties that are difficult to predict. Although we believe that the expectations reflected in these forward-looking statements are reasonable as of the date made, actual results may prove to be materially different from the results expressed or implied by the forward-looking statements. When considering forward-looking statements, you should keep in mind the risk factors and other cautionary statements described in the "Risk Factors" section of this prospectus.

There are or may be important factors that could cause our actual results to differ materially from those indicated in these forward-looking statements, including, but not limited to, the following: