Registration Statement - Specified Transactions (Form S-3)

As filed with the Securities and Exchange Commission on October 10, 2025

Registration No. 333-         

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM S-3

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

SELLAS Life Sciences Group, Inc.

(Exact name of registrant as specified in its charter)

Delaware (State or other jurisdiction of incorporation or organization)

20-8099512 (I.R.S. Employer Identification Number)

7 Times Square, Suite 2503

New York, NY 10036

(646) 200-5278

(Address, including zip code, and telephone number, including area code, of registrant's principal executive offices)

Angelos M. Stergiou, M.D., ScD h.c.

President and Chief Executive Officer

7 Times Square, Suite 2503

New York, NY 10036

(646) 200-5278

(Name, address, including zip code, and telephone number, including area code, of agent for service)

Copies to:

Daniel A. Bagliebter, Esq.

Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C.

919 Third Avenue

New York, New York 10022

(212) 935-3000

Approximate date of commencement of proposed sale to the public: From time to time after the effective date of this registration statement.

If the only securities being registered on this Form are being offered pursuant to dividend or interest reinvestment plans, please check the following box: ¨

If any of the securities being registered on this Form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, other than securities offered only in connection with dividend or interest reinvestment plans, check the following box: x

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, please check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. ¨

If this Form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. ¨

If this Form is a registration statement pursuant to General Instruction I.D. or a post-effective amendment thereto that shall become effective upon filing with the Commission pursuant to Rule 462(e) under the Securities Act, check the following box. ¨

If this Form is a post-effective amendment to a registration statement filed pursuant to General Instruction I.D. filed to register additional securities or additional classes of securities pursuant to Rule 413(b) under the Securities Act, check the following box. ¨

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, a smaller reporting company, or an emerging growth company. See the definitions of "large accelerated filer," "accelerated filer," "smaller reporting company" and "emerging growth company" in Rule 12b-2 of the Exchange Act.

Large accelerated filer ¨		Accelerated filer ¨
Non-accelerated filer x		Smaller reporting company x
Emerging growth company ¨

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of the Securities Act. ¨

The Registrant hereby amends this registration statement on such date or dates as may be necessary to delay its effective date until the Registrant shall file a further amendment that specifically states that this registration statement shall thereafter become effective in accordance with Section 8(a) of the Securities Act of 1933, as amended, or until this registration statement shall become effective on such date as the Securities and Exchange Commission, acting pursuant to said Section 8(a), may determine.

The information in this prospectus is not complete and may be changed. A registration statement relating to these securities has been filed with the Securities and Exchange Commission. We may not sell these securities or accept an offer to buy these securities until the registration statement is effective. This prospectus is not an offer to sell these securities and it is not soliciting offers to buy these securities in any jurisdiction where the offer or sale is not permitted.

PRELIMINARY PROSPECTUS - SUBJECT TO COMPLETION, DATED OCTOBER 10, 2025

SELLAS LIFE SCIENCES GROUP, INC.

19,685,040 Shares of Common Stock

This prospectus covers the offer and resale by the selling stockholders identified in this prospectus of up to an aggregate of 19,685,040 shares of our common stock, $0.0001 par value per share (the "Common Stock"), consisting of 19,685,040 shares of Common Stock issuable upon the exercise of Warrants to purchase shares of Common Stock (collectively, the "Warrants"). We are registering the shares issuable upon exercise of the Warrants on behalf of the selling stockholders, to be offered and sold by them from time to time.

We are not selling any shares of Common Stock under this prospectus and will not receive any proceeds from the sale by the selling stockholders of such shares. We are paying the cost of registering the shares of Common Stock covered by this prospectus as well as various related expenses. The selling stockholders are responsible for all selling commissions, transfer taxes and other costs related to the offer and sale of their shares.

Sales of the shares by the selling stockholders may occur at fixed prices, at market prices prevailing at the time of sale, at prices related to prevailing market prices, at negotiated prices and/or at varying prices determined at the time of sale. The selling stockholders may sell shares directly or to or through underwriters, broker-dealers or agents, who may receive compensation in the form of discounts, concessions or commissions from the selling stockholders, the purchasers of the shares, or both. The selling stockholders may sell any, all or none of the securities offered by this prospectus, and we do not know when or in what amount the selling stockholders may sell their shares of Common Stock hereunder following the effective date of the registration statement of which this prospectus forms a part. We provide more information about how the selling stockholders may sell or otherwise dispose of their shares of Common Stock in the section titled "Plan of Distribution" on page 9.

Our Common Stock is listed on The Nasdaq Capital Market under the symbol "SLS." On October 9, 2025, the last reported sale price of our Common Stock was $1.8950 per share.

Investing in our securities involves a high degree of risk. You should carefully consider the risks described under "Risk Factors" beginning on page 6 of this prospectus, and under similar headings in the other documents that are incorporated by reference into this prospectus.

Neither the Securities and Exchange Commission nor any state securities commission 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.

The date of this prospectus is          , 2025

TABLE OF CONTENTS

	Page
ABOUT THIS PROSPECTUS	ii
SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS	iii
PROSPECTUS SUMMARY	1
THE OFFERING	5
RISK FACTORS	6
USE OF PROCEEDS	7
SELLING STOCKHOLDERS	7
PLAN OF DISTRIBUTION	9
LEGAL MATTERS	11
EXPERTS	11
WHERE YOU CAN FIND MORE INFORMATION	11
INCORPORATION OF CERTAIN INFORMATION BY REFERENCE	11

ABOUT THIS PROSPECTUS

This prospectus is part of a registration statement on Form S-3 that we filed with the Securities and Exchange Commission (the "SEC") pursuant to which the selling stockholders may sell, from time to time in one or more offerings, the Common Stock described in this prospectus.

We have not authorized anyone to provide you with information other than the information that we have provided or incorporated by reference in this prospectus and your reliance on any unauthorized information or representation is at your own risk. This prospectus may be used only in jurisdictions where offers and sales of these securities are permitted. You should assume that the information appearing in this prospectus is accurate only as of the date of this prospectus and that any information we have incorporated by reference is accurate only as of the date of the document incorporated by reference, regardless of the time of delivery of this prospectus, or any sale of our Common Stock. Our business, financial condition and results of operations may have changed since those dates.

Unless otherwise stated, all references in this prospectus to "we," "us," "our," "SELLAS," the "Company" and similar designations refer to SELLAS Life Sciences Group, Inc. This prospectus contains references to trademarks belonging to other entities. Solely for convenience, trademarks and trade names referred to in this prospectus, including logos, artwork and other visual displays, may appear without the ® or ™ symbols, but such references are not intended to indicate, in any way, that the applicable licensor will not assert, to the fullest extent under applicable law, its rights to these trademarks and trade names. We do not intend our use or display of other companies' trade names or trademarks to imply a relationship with, or endorsement or sponsorship of us by, any other companies.

A prospectus supplement may add to, update or change the information contained in this prospectus. You should read both this prospectus and any applicable prospectus supplement together with additional information described below under the heading "Where You Can Find Additional Information."

SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS

This prospectus and the documents incorporated by reference contain forward-looking statements about us and our industry that involve substantial risks and uncertainties. All statements other than statements of historical facts contained in this prospectus and the documented incorporated by reference, including statements regarding our future financial condition, business strategy and plans, and objectives of management for future operations, are forward-looking statements. In some cases, you can identify forward-looking statements by terminology such as "aim," "anticipate," "assume," "believe," "contemplate," "continue," "could," "design," "due," "estimate," "expect," "goal," "intend," "may," "objective," "plan," "predict," "positioned," "potential," "seek," "should," "target," "will," "would" and other similar expressions that are predictions of or indicate future events and future trends, or the negative of these terms or other comparable terminology.

We have based these forward-looking statements largely on our current expectations and projections about future events and financial trends that we believe may affect our financial condition, results of operations, business strategy and financial needs. These forward-looking statements are subject to a number of known and unknown risks, uncertainties and assumptions, including risks described in the section titled "Risk Factors" contained in our most recent Annual Report on Form 10-K and Quarterly Reports on Form 10-Q and incorporated by reference in this prospectus, as the same may be amended, supplemented or superseded by the risks and uncertainties described under similar headings in the other documents that are filed after the date hereof and incorporated by reference into this prospectus, regarding, among other things:

·

our future financial and business performance;

·

strategic plans for our business and product candidates;

·

our ability to develop or commercialize products;

·

the expected results and timing of clinical trials and nonclinical studies;

·

our ability to comply with the terms of our license agreements;

·

developments and projections relating to our competitors and industry;

·

our expectations regarding our ability to obtain, develop and maintain intellectual property protection and not infringe on the rights of others;

·

our ability to retain and attract highly-skilled executive officers and employees;

·

our future capital requirements and the timing of those requirements and sources and uses of cash;

·

our ability to obtain funding for our operations;

·

changes in the assumptions underlying our expectations regarding our future business or business model;

·

risks associated with preclinical or clinical development and trials;

·

our ability to develop, manufacture and commercialize product candidates;

·

general economic, financial, legal, political and business conditions and changes in domestic and foreign markets;

·

changes in applicable laws or regulations;

·

the impact of natural disasters, including climate change, and the impact of health epidemics, on our business;

·

the size and growth potential of the markets for our products, and our ability to serve those markets;

·

market acceptance of our planned products;

·

our ability to raise capital; and

·

the possibility that we may be adversely affected by other economic, business, and/or competitive factors.

These risks are not exhaustive. Other sections of this prospectus, or the documents incorporated herein by reference may include additional factors that could harm our business and financial performance. Moreover, we operate in a very competitive and rapidly changing environment. New risk factors emerge from time to time, and it is not possible for our management to predict all risk factors, nor can we assess the impact of all factors on our business or the extent to which any factor, or combination of factors, may cause actual results to differ materially from those contained in, or implied by, any forward-looking statements.

You should not rely upon forward-looking statements as predictions of future events. We cannot assure you that the events and circumstances reflected in the forward-looking statements will be achieved or occur. Although we believe that the expectations reflected in the forward-looking statements are reasonable, we cannot guarantee future results, levels of activity, performance or achievements. Except as required by law, we undertake no obligation to update publicly any forward-looking statements for any reason after the date of this prospectus or to conform these statements to actual results or to changes in our expectations.

You should carefully read this prospectus, and the applicable prospectus supplement, together with the information incorporated herein by reference as described under the heading "Incorporation by Reference," as well as the documents filed as exhibits to the registration statement of which this prospectus is a part with the understanding that our actual future results, levels of activity, performance and achievements may be materially different from what we expect. We qualify all of our forward-looking statements by these cautionary statements.

Except as required by law, we assume no obligation to update these forward-looking statements publicly, or to update the reasons actual results could differ materially from those anticipated in these forward-looking statements, even if new information becomes available in the future.

THE OFFERING

Common stock offered by the selling stockholders		Up to 19,685,040 shares of Common Stock underlying the Inducement Warrants.
Terms of the offering		The selling stockholders will determine when and how they will sell the Common Stock offered in this prospectus, as described in "Plan of Distribution."
Use of proceeds		We will not receive any proceeds from the sale of shares of our Common Stock by the selling stockholders. We may receive up to approximately $37,007,875.20 in aggregate gross proceeds from the exercise of the Inducement Warrants, if the Inducement Warrants are exercised for cash (and not a cashless basis), based on the per share exercise price of the Inducement Warrants. Any proceeds we receive from the exercise of the Inducement Warrants will be used for working capital and general corporate purposes.
Risk factors		An investment in our securities involves a high degree of risk. See the section entitled "Risk Factors" of this prospectus and the similarly titled sections in the documents incorporated by reference into this prospectus.
Nasdaq Capital Market Symbol		SLS

The selling stockholders named in this prospectus may offer and sell up to 19,685,040 shares of our Common Stock.

The selling stockholders are prohibited, subject to certain exceptions, from exercising the Inducement Warrants to the extent that immediately prior to or after giving effect to such exercise, the selling stockholders, together with their respective affiliates and other attribution parties, would own more than 4.99% of the total number of shares of Common Stock then issued and outstanding, which percentage may be increased or decreased at the selling stockholder's election to a percentage not to exceed 9.99%, provided that any increase will require 61 days' notice to the Company.

Our Common Stock is currently listed on The Nasdaq Capital Market under the symbol "SLS."

Shares of Common Stock that may be offered under this prospectus will be fully paid and non-assessable. We will not receive any of the proceeds of sales by the selling stockholders of any of the Common Stock covered by this prospectus. Throughout this prospectus, when we refer to the shares of our Common Stock being registered on behalf of the selling stockholders for offer and resale, we are referring to the shares of Common Stock issued to the selling stockholders in connection with the exercise of warrants issued in the transaction as described above. When we refer to the selling stockholders in this prospectus, we are referring to the selling stockholders identified in this prospectus and, as applicable, their permitted transferees or other successors-in-interest that may be identified in a supplement to this prospectus or, if required, a post-effective amendment to the registration statement of which this prospectus is a part.

RISK FACTORS

Investing in our securities involves risks. Before you make a decision to buy our securities, in addition to the risks and uncertainties discussed above under "Special Note Regarding Forward-Looking Statements," you should carefully consider the specific risks incorporated by reference in this prospectus to our most recent Annual Report on Form 10-K and any subsequent Quarterly Reports on Form 10-Q or Current Reports on Form 8-K, and all other information contained or incorporated by reference into this prospectus, as updated by our subsequent filings under the Exchange Act, and the risk factors and other information contained in any applicable prospectus supplement and any applicable free writing prospectus before acquiring any such securities. If any of these risks actually occur, it may materially harm our business, financial condition, liquidity and results of operations. As a result, the market price of our securities could decline, and you could lose all or part of your investment. Additionally, the risks and uncertainties incorporated by reference in this prospectus or any prospectus supplement are not the only risks and uncertainties that we face. Additional risks and uncertainties not presently known to us or that we currently believe to be immaterial, but that could later become material, may also adversely affect our business.

USE OF PROCEEDS

We will not receive any of the proceeds from the sale of securities by the selling stockholder pursuant to this prospectus. We may receive up to approximately $37,007,875.20 in aggregate gross proceeds from the exercise of the Inducement Warrants, if the Inducement Warrants are exercised for cash (and not a cashless basis), based on the per share exercise price of the Inducement Warrants. Any proceeds we receive from the exercise of the Inducement Warrants will be used for working capital and general corporate purposes.

SELLING STOCKHOLDERS

The shares of Common Stock being offered by the selling stockholders are those issuable to the selling stockholders, upon exercise of the Inducement Warrants. For additional information regarding the issuances of those shares of Common Stock and the Inducement Warrants, see "Prospectus Summary-Recent Developments" above. We are registering the shares of Common Stock issuable upon exercise of the Inducement Warrants in order to permit the selling stockholders to offer the shares for resale from time to time. Except for the ownership of the shares of Common Stock and the Inducement Warrants, the selling stockholders have not had any material relationship with us within the past three years. The selling stockholders may sell all, some or none of their shares in this offering. See "Plan of Distribution."

The table below lists the selling stockholders and other information regarding the beneficial ownership (as determined under Section 13(d) of the Exchange Act and the rules and regulations thereunder) of the shares of Common Stock by each of the selling stockholders. Under the terms of the Inducement Warrants, a selling stockholder may not exercise the Inducement Warrants to the extent such exercise would cause such selling stockholder, together with its affiliates, to beneficially own a number of shares of Common Stock that would exceed 4.99% of our then outstanding shares of Common Stock following such exercise, which percentage may be increased or decreased at the selling stockholder's election to a percentage not to exceed 9.99%, provided that any increase will require 61 days' notice to the Company, excluding for purposes of such determination shares of Common Stock issuable upon exercise of the Inducement Warrants which have not been exercised. The number of shares beneficially owned prior to the offering in the second column (i) is based on 125,078,525 shares of our Common Stock outstanding as of October 1, 2025, and (ii) takes into account the beneficial ownership limitations described above.

The third column lists the shares of Common Stock being offered by this prospectus by the selling stockholders. In accordance with the terms of our registration rights agreement with the selling stockholders, this prospectus generally covers the resale of the maximum number of shares of Common Stock issuable upon exercise of the Inducement Warrants, without regard to any limitations on the exercise of the Inducement Warrants, and this registration statement registers the maximum number of shares of Common Stock as shall from time to time be necessary to effect the exercise of the Inducement Warrants then outstanding without giving effect to any limitation on exercise included in the Inducement Warrants. The number and percentage of shares beneficially owned after the offering in the fourth and fifth columns (i) is based on 125,078,525 shares of our Common Stock outstanding as of October 1, 2025, (ii) assumes the sale of all of the shares offered by the selling stockholders pursuant to this prospectus and (iii) takes into account the beneficial ownership limitations described above. This table was prepared based on information furnished to us by the selling stockholders.

Name of Selling Stockholder		Number of Shares of Common Stock Owned Prior to Offering				Maximum Number of Shares of Common Stock to be Sold Pursuant to this Prospectus				Number of Shares of Common Stock Owned After Offering(1)				Percentage of Shares of Common Stock Owned After Offering if Greater than 1%
Anson Investments Master Fund LP			37,478,027	(2)			15,354,331				22,123,696				4.99	%
Anson East Master Fund LP			10,570,727	(3)			4,330,709				6,240,018				4.80	%

(1)

Assumes that all Common Stock being registered under the registration statement of which this prospectus forms a part are sold in this offering, and that none of the selling stockholders acquire additional Common Stock after the date of this prospectus and prior to completion of this offering.

(2)

Consists of (i) 15,354,331 shares of Common Stock being registered hereby, (ii) 4,680,000 shares of Common Stock held as of October 1, 2025 and (iii) 17,443,696 shares of Common Stock issuable upon the exercise of previously held warrants. Anson Advisors Inc and Anson Funds Management LP, the Co-Investment Advisers of Anson Investments Master Fund LP ("Anson"), hold voting and dispositive power over the shares of Common Stock held by Anson. Tony Moore is the managing member of Anson Management GP LLC, which is the general partner of Anson Funds Management LP. Moez Kassam and Amin Nathoo are directors of Anson Advisors Inc. Mr. Moore, Mr. Kassam and Mr. Nathoo each disclaim beneficial ownership of these shares of Common Stock except to the extent of their pecuniary interest therein. The principal business address of Anson is Maple Corporate Services Limited, PO Box 309, Ugland House, Grand Cayman, KYI-1104, Cayman Islands.

(3)

Consists of (i) 4,330,709 shares of Common Stock being registered hereby, (ii) 1,320,000 shares of Common Stock held as of October 1, 2025 and (iii) 4,920,018 shares of Common Stock issuable upon the exercise of previously held warrants. Anson Advisors Inc and Anson Funds Management LP, the Co-Investment Advisers of Anson East Master Fund LP ("Anson East"), hold voting and dispositive power over the shares of Common Stock held by Anson East. Tony Moore is the managing member of Anson Management GP LLC, which is the general partner of Anson Funds Management LP. Moez Kassam and Amin Nathoo are directors of Anson Advisors Inc. Mr. Moore, Mr. Kassam and Mr. Nathoo each disclaim beneficial ownership of these shares of Common Stock except to the extent of their pecuniary interest therein. The principal business address of Anson East is Maple Corporate Services Limited, PO Box 309, Ugland House, Grand Cayman, KYI-1104, Cayman Islands.

PLAN OF DISTRIBUTION

We are registering the shares of Common Stock issuable upon exercise of the Inducement Warrants to permit the resale of these shares of Common Stock from time to time after the date of this prospectus. We will not receive any of the proceeds from the sale by the selling stockholders of the shares of Common Stock. We will bear all fees and expenses incident to our obligation to register the shares of Common Stock.

The selling stockholders may sell all or a portion of the shares of Common Stock they acquire upon exercise of the Inducement Warrants beneficially owned by them and offered hereby from time to time directly or through one or more underwriters, broker-dealers or agents. If the shares of Common Stock are sold through underwriters or broker-dealers, the selling stockholders will be responsible for underwriting discounts or commissions or agent's commissions. The shares of Common Stock may be sold in one or more transactions at fixed prices, at prevailing market prices at the time of the sale, at varying prices determined at the time of sale, or at negotiated prices. These sales may be effected in transactions, which may involve crosses or block transactions,

	·	on any national securities exchange or quotation service on which the securities may be listed or quoted at the time of sale;
	·	in the over-the-counter market;
	·	in transactions otherwise than on these exchanges or systems or in the over-the-counter market;
	·	through the writing of options, whether such options are listed on an options exchange or otherwise;
	·	ordinary brokerage transactions and transactions in which the broker-dealer solicits purchasers;
	·	block trades in which the broker-dealer will attempt to sell the shares as agent but may position and resell a portion of the block as principal to facilitate the transaction;
	·	purchases by a broker-dealer as principal and resale by the broker-dealer for its account;
	·	an exchange distribution in accordance with the rules of the applicable exchange;
	·	privately negotiated transactions;
	·	short sales;
	·	sales pursuant to Rule 144;
	·	broker-dealers may agree with the selling stockholders to sell a specified number of such shares at a stipulated price per share;
	·	a combination of any such methods of sale; and
	·	any other method permitted pursuant to applicable law.

If the selling stockholders effect such transactions by selling shares of Common Stock to or through underwriters, broker-dealers or agents, such underwriters, broker-dealers or agents may receive commissions in the form of discounts, concessions or commissions from the selling stockholders or commissions from purchasers of the shares of Common Stock for whom they may act as agent or to whom they may sell as principal (which discounts, concessions or commissions as to particular underwriters, broker-dealers or agents may be in excess of those customary in the types of transactions involved). In connection with sales of the shares of Common Stock or otherwise, the selling stockholders may enter into hedging transactions with broker-dealers, which may in turn engage in short sales of the shares of Common Stock in the course of hedging in positions they assume. The selling stockholders may also sell shares of Common Stock short and deliver shares of Common Stock covered by this prospectus to close out short positions and to return borrowed shares in connection with such short sales. The selling stockholders may also loan or pledge shares of Common Stock to broker-dealers that in turn may sell such shares.

The selling stockholders may pledge or grant a security interest in some or all of the Inducement Warrants or shares of Common Stock they acquire upon the exercise of the Inducement Warrants owned by them and, if they default in the performance of their secured obligations, the pledgees or secured parties may offer and sell the shares of Common Stock from time to time pursuant to this prospectus or any amendment to this prospectus under Rule 424(b)(3) or other applicable provision of the Securities Act of 1933, as amended, amending, if necessary, the list of selling stockholders to include the pledgee, transferee or other successors in interest as selling stockholders under this prospectus. The selling stockholders also may transfer and donate the shares of Common Stock in other circumstances in which case the transferees, donees, pledgees or other successors in interest will be the selling beneficial owners for purposes of this prospectus.

The selling stockholders and any broker-dealer participating in the distribution of the shares of Common Stock may be deemed to be "underwriters" within the meaning of the Securities Act, and any commission paid, or any discounts or concessions allowed to, any such broker-dealer may be deemed to be underwriting commissions or discounts under the Securities Act. At the time a particular offering of the shares of Common Stock is made, a prospectus supplement, if required, will be distributed which will set forth the aggregate amount of shares of Common Stock being offered and the terms of the offering, including the name or names of any broker-dealers or agents, any discounts, commissions and other terms constituting compensation from the selling stockholders and any discounts, commissions or concessions allowed or reallowed or paid to broker-dealers.

Under the securities laws of some states, the shares of Common Stock may be sold in such states only through registered or licensed brokers or dealers. In addition, in some states the shares of Common Stock may not be sold unless such shares have been registered or qualified for sale in such state or an exemption from registration or qualification is available and is complied with.

There can be no assurance that any selling stockholder will sell any or all of the shares of Common Stock registered pursuant to the registration statement, of which this prospectus forms a part.

The selling stockholders and any other person participating in such distribution will be subject to applicable provisions of the Securities Exchange Act of 1934, as amended, and the rules and regulations thereunder, including, without limitation, Regulation M of the Exchange Act, which may limit the timing of purchases and sales of any of the shares of Common Stock by the selling stockholders and any other participating person. Regulation M may also restrict the ability of any person engaged in the distribution of the shares of Common Stock to engage in market-making activities with respect to the shares of Common Stock. All of the foregoing may affect the marketability of the shares of Common Stock and the ability of any person or entity to engage in market-making activities with respect to the shares of Common Stock.

We will pay all expenses of the registration of the shares of Common Stock issuable upon exercise of the Inducement Warrants, estimated to be $54,063.21 in total, including, without limitation, SEC filing fees and expenses of compliance with state securities or "blue sky" laws; provided, however, that a selling stockholder will pay all underwriting discounts and selling commissions, if any. We will indemnify the selling stockholders against liabilities, including some liabilities under the Securities Act, in accordance with the registration rights agreements, or the selling stockholders will be entitled to contribution. We may be indemnified by the selling stockholders against civil liabilities, including liabilities under the Securities Act, that may arise from any written information furnished to us by the selling stockholder specifically for use in this prospectus, in accordance with the related registration rights agreement, or we may be entitled to contribution.

Once sold under the registration statement, of which this prospectus forms a part, the shares of Common Stock will be freely tradable in the hands of persons other than our affiliates.

LEGAL MATTERS

The validity of the securities we are offering will be passed upon for us by Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C., New York, New York.

EXPERTS

The consolidated financial statements of the Company incorporated in this Prospectus by reference from the Annual Report on Form 10-K of the Company for the year ended December 31, 2024, have been audited by Baker Tilly US, LLP (formerly, Moss Adams LLP), an independent registered public accounting firm, as stated in their report (which report expresses an unqualified opinion and includes an explanatory paragraph related to a going concern uncertainty), which is incorporated herein by reference. Such consolidated financial statements are incorporated by reference in reliance upon the report of such firm given their authority as experts in accounting and auditing.

WHERE YOU CAN FIND MORE INFORMATION

We have filed a registration statement on Form S-3, including exhibits, under the Securities Act with respect to the securities offered by this prospectus. This prospectus does not contain all of the information included in the registration statement. For further information pertaining to us and our securities, you should refer to the registration statement and our exhibits.

In addition, we file annual, quarterly and current reports, proxy statements and other information with the SEC. Our filings with the SEC are available to the public on a website maintained by the SEC located at www.sec.gov. We also maintain a website at www.sellaslifesciences.com. Through our website, we make available, free of charge, annual, quarterly and current reports, proxy statements and other information as soon as reasonably practicable after they are electronically filed with, or furnished to, the SEC. The information contained on, or that may be accessed through, our website is not part of, and is not incorporated into, this prospectus. We include our website address in this prospectus only as an inactive textual reference. Information contained in our website does not constitute a part of this prospectus or our other filings with the SEC.

INCORPORATION OF CERTAIN INFORMATION BY REFERENCE

The SEC allows us to "incorporate by reference" information that we file with them. Incorporation by reference allows us to disclose important information to you by referring you to those other documents. The information incorporated by reference is an important part of this prospectus, and information that we file later with the SEC will automatically update and supersede this information. We filed a registration statement on Form S-3 under the Securities Act with the SEC with respect to the securities we may offer pursuant to this prospectus. This prospectus omits certain information contained in the registration statement, as permitted by the SEC. You should refer to the registration statement, including the exhibits, for further information about us and the securities we may offer pursuant to this prospectus. Statements in this prospectus regarding the provisions of certain documents filed with, or ,incorporated by reference in, the registration statement are not necessarily complete and each statement is qualified in all respects by that reference. Copies of all or any part of the registration statement, including the documents incorporated by reference or the exhibits, may be obtained upon payment of the prescribed rates at the offices of the SEC listed above in "Where You Can Find More Information." The documents we are incorporating by reference are:

·

our Annual Report on Form 10-K for the year ended December 31, 2024, filed with the SEC on March 20, 2025;

·

our Quarterly Reports on Form 10-Q for the fiscal quarters ended March 31, 2025 and June 30, 2025, filed with the SEC on May 13, 2025 and August 12, 2025 respectively;

·

our Current Reports on Form 8-K filed with the SEC on January 16, 2025, January 29, 2025, June 6, 2025, June 18, 2025 and September 11, 2025, except for the information furnished under Items 2.02 or 7.01 and the exhibits thereto;

·

the description of our common stock set forth in our registration statement on Form 8-A, filed with the SEC on February 8, 2008, as amended on February 12, 2008, including any further amendments thereto or reports filed for the purposes of updating this description; and

·

all documents filed by us with the SEC pursuant to Sections 13(a), 13(c), 14 and 15(d) of the Exchange Act (other than Current Reports furnished under Item 2.02 or Item 7.01 of Form 8-K and exhibits filed on such forms that are related to such items) after the date of this prospectus and prior to the termination of this offering, provided that all documents "furnished" by the Company to the SEC and not "filed" are not deemed incorporated by reference herein.

In addition, all reports and other documents filed by us pursuant to Sections 13(a), 13(c), 14 and 15(d) of the Exchange Act after the date of the initial registration statement and prior to effectiveness of the registration statement shall be deemed to be incorporated by reference into this prospectus and deemed to be a part of this prospectus from the date of filing of such reports and documents. Notwithstanding the foregoing, we are not incorporating by reference any information furnished pursuant to Items 2.02 or 7.01 and the exhibits filed on such forms that are related to such items.

Any statement contained in a document incorporated by reference herein shall be deemed to be modified or superseded for all purposes to the extent that a statement contained in this prospectus or in any other subsequently filed document which is also incorporated or deemed to be incorporated by reference, modifies or supersedes such statement. Any statement so modified or superseded shall not be deemed, except as so modified or superseded, to constitute a part of this prospectus. You may request a copy of these filings (other than an exhibit to a filing unless that exhibit is specifically incorporated by reference into that filing) at no cost by writing, telephoning or e-mailing us at the following address or telephone number:

SELLAS Life Sciences Group, Inc.,

Attention: Corporate Secretary

7 Times Square

Suite 2503

New York, New York 10036

(646) 200-5278

You should rely only on the information incorporated by reference or provided in this prospectus or any prospectus supplement. We have not authorized anyone to provide you with different information. You should not assume that the information contained in this prospectus or the accompanying prospectus supplement is accurate on any date subsequent to the date set forth on the front of the document or that any information we have incorporated by reference is correct on any date subsequent to the date of the document incorporated by reference, even though this prospectus and any accompanying prospectus supplement is delivered or securities are sold on a later date.

SELLAS Life Sciences Group, Inc.

19,685,040 Shares of Common Stock

        , 2025

PART II

INFORMATION NOT REQUIRED IN PROSPECTUS

Item 14. Other Expenses of Issuance and Distribution

The following table sets forth the costs and expenses payable in connection with the sale and distribution of the securities being registered. All amounts are estimates except the Securities and Exchange Commission (the "SEC") registration fee.

SEC Registration Fee		$	5,063.21
Legal Fees and Expenses		$	35,000
Accounting Fees and Expenses		$	9,000
Miscellaneous		$	5,000
Total		$	54,063.21

Item 15. Indemnification of Directors and Officers

As permitted by Section 102 of the DGCL, we have adopted provisions in our amended and restated certificate of incorporation and amended and restated bylaws, each as amended, which limit or eliminate the personal liability of our directors for a breach of their fiduciary duty of care as a director. The duty of care generally requires that, when acting on behalf of the corporation, directors exercise an informed business judgment based on all material information reasonably available to them. Consequently, a director will not be personally liable to us or our stockholders for monetary damages for breach of fiduciary duty as a director, except for liability for:

·

any breach of the director's duty of loyalty to us or our stockholders;

·

any act or omission not in good faith or that involves intentional misconduct or a knowing violation of law;

·

any act related to unlawful stock repurchases, redemptions or other distributions or payment of dividends; or

·

any transaction from which the director derived an improper personal benefit.

These limitations of liability do not affect the availability of equitable remedies such as injunctive relief or rescission. Our amended and restated certificate of incorporation also authorizes us to indemnify our officers, directors and other agents to the fullest extent permitted under Delaware law.

As permitted by Section 145 of the DGCL, our amended and restated bylaws, as amended, provide that:

·

we may indemnify our directors, officers and employees to the fullest extent permitted by the DGCL, subject to limited exceptions;

·

we may advance expenses to our directors, officers and employees in connection with a legal proceeding to the fullest extent permitted by the DGCL, subject to limited exceptions; and

·

the rights provided in our amended and restated bylaws are not exclusive.

Our amended and restated certificate of incorporation and bylaws, each as amended, provide for the indemnification provisions described above and elsewhere herein. We have entered into separate indemnification agreements with our directors and officers that may be broader than the specific indemnification provisions contained in the DGCL. These indemnification agreements generally require us, among other things, to indemnify our officers and directors against liabilities that may arise by reason of their status or service as directors or officers, other than liabilities arising from willful misconduct. These indemnification agreements also generally require us to advance any expenses incurred by the directors or officers as a result of any proceeding against them as to which they could be indemnified. In addition, we have purchased a policy of directors' and officers' liability insurance that insures our directors and officers against the cost of defense, settlement or payment of a judgment in some circumstances. These indemnification provisions and the indemnification agreements may be sufficiently broad to permit indemnification of our officers and directors for liabilities, including reimbursement of expenses incurred, arising under the Securities Act.

We have entered into indemnification agreements with our directors and executive officers, in addition to the indemnification provided for in our amended and restated certificate of incorporation and amended and restated bylaws, and intend to enter into indemnification agreements with any new directors and executive officers in the future.

We have purchased and currently intend to maintain insurance on behalf of each and every person who is or was a director or officer of our company against any loss arising from any claim asserted against him or her and incurred by him or her in any such capacity, subject to certain exclusions.

Item 16. Exhibits

Exhibit Number		Description
4.1		Form of Inducement Warrant (incorporated by reference to Exhibit 4.1 to our Current Report on Form 8-K, filed on September 11, 2025).
5.1*		Opinion of Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C.
10.1		Form of Inducement Agreement (incorporated by reference to Exhibit 10.1 to our Current Report on Form 8-K, filed on September 11, 2025).
23.1*		Consent of Baker Tilly US, LLP, Independent Registered Public Accounting Firm
23.2*		Consent of Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C. (contained in Exhibit 5.1 hereto).
24.1*		Powers of Attorney (included in the signature page of this registration statement).
107*		Filing Fee Table

* Filed herewith.

Item 17. Undertakings

(a)	The undersigned registrant hereby undertakes:

(1)

To file, during any period in which offers or sales are being made, a post-effective amendment to this registration statement:

(i)

To include any prospectus required by section 10(a)(3) of the Securities Act of 1933;

(ii)

To reflect in the prospectus any facts or events arising after the effective date of the registration statement (or the most recent post-effective amendment thereof) which, individually or in the aggregate, represent a fundamental change in the information set forth in the registration statement. Notwithstanding the foregoing, any increase or decrease in volume of securities offered (if the total dollar value of securities offered would not exceed that which was registered) and any deviation from the low or high end of the estimated maximum offering range may be reflected in the form of prospectus filed with the Commission pursuant to Rule 424(b) if, in the aggregate, the changes in volume and price represent no more than a 20% change in the maximum aggregate offering price set forth in the "Calculation of Registration Fee" table in the effective registration statement; and

(iii)

To include any material information with respect to the plan of distribution not previously disclosed in the registration statement or any material change to such information in the registration statement;

provided, however, that paragraphs (a)(1)(i), (a)(1)(ii) and (a)(1)(iii) do not apply if the information required to be included in a post-effective amendment by those paragraphs is contained in reports filed with or furnished to the Commission by the registrant pursuant to section 13 or section 15(d) of the Securities Exchange Act of 1934 that are incorporated by reference in the registration statement, or is contained in a form of prospectus filed pursuant to Rule 424(b) that is part of the registration statement.

(2)

That, for the purpose of determining any liability under the Securities Act of 1933, each such post-effective amendment shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.

(3)

To remove from registration by means of a post-effective amendment any of the securities being registered which remain unsold at the termination of the offering.

(4)

That, for the purpose of determining liability under the Securities Act of 1933 to any purchaser:

(i)

Each prospectus filed by the registrant pursuant to Rule 424(b)(3) shall be deemed to be part of the registration statement as of the date the filed prospectus was deemed part of and included in the registration statement; and

(ii)

Each prospectus required to be filed pursuant to Rule 424(b)(2), (b)(5), or (b)(7) as part of a registration statement in reliance on Rule 430B relating to an offering made pursuant to Rule 415(a)(1)(i), (vii), or (x) for the purpose of providing the information required by section 10(a) of the Securities Act of 1933 shall be deemed to be part of and included in the registration statement as of the earlier of the date such form of prospectus is first used after effectiveness or the date of the first contract of sale of securities in the offering described in the prospectus. As provided in Rule 430B, for liability purposes of the issuer and any person that is at that date an underwriter, such date shall be deemed to be a new effective date of the registration statement relating to the securities in the registration statement to which that prospectus relates, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof. Provided, however, that no statement made in a registration statement or prospectus that is part of the registration statement or made in a document incorporated or deemed incorporated by reference into the registration statement or prospectus that is part of the registration statement will, as to a purchaser with a time of contract of sale prior to such effective date, supersede or modify any statement that was made in the registration statement or prospectus that was part of the registration statement or made in any such document immediately prior to such effective date.

(5)

That, for the purpose of determining liability of the registrant under the Securities Act to any purchaser in the initial distribution of the securities, the undersigned registrant undertakes that in a primary offering of securities of the undersigned registrant pursuant to this registration statement, regardless of the underwriting method used to sell the securities to the purchaser, if the securities are offered or sold to such purchaser by means of any of the following communications, the undersigned registrant will be a seller to the purchaser and will be considered to offer or sell such securities to such purchaser:

(i)

Any preliminary prospectus or prospectus of the undersigned registrant relating to the offering required to be filed pursuant to Rule 424;

(ii)

Any free writing prospectus relating to the offering prepared by or on behalf of the undersigned registrant or used or referred to by the undersigned registrant;

(iii)

The portion of any other free writing prospectus relating to the offering containing material information about the undersigned registrant or its securities provided by or on behalf of the undersigned registrant; and

(iv)

Any other communication that is an offer in the offering made by the undersigned registrant to the purchaser.

(b)

The undersigned registrant hereby undertakes that, for purposes of determining any liability under the Securities Act of 1933, each filing of the registrant's annual report pursuant to section 13(a) or section 15(d) of the Securities Exchange Act of 1934 (and, where applicable, each filing of an employee benefit plan's annual report pursuant to section 15(d) of the Securities Exchange Act of 1934) that is incorporated by reference in the registration statement shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.

(c)

Insofar as indemnification for liabilities arising under the Securities Act of 1933 may be permitted to directors, officers and controlling persons of the registrant, the registrant has been advised that in the opinion of the Securities and Exchange Commission such indemnification is against public policy as expressed in the Act and is, therefore, unenforceable. In the event that a claim for indemnification against such liabilities (other than the payment by the registrant of expenses incurred or paid by a director, officer or controlling person of the registrant in the successful defense of any action, suit or proceeding) is asserted by such director, officer or controlling person in connection with the securities being registered, the registrant will, unless in the opinion of its counsel the matter has been settled by controlling precedent, submit to a court of appropriate jurisdiction the question whether such indemnification by it is against public policy as expressed in the Act and will be governed by the final adjudication of such issue.

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, the registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form S-3 and has duly caused this registration statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the city of New York, state of New York, on October 10, 2025.

	SELLAS LIFE SCIENCES GROUP, INC.
	By:	/s/ Angelos M. Stergiou
		Angelos M. Stergiou, M.D., ScD h.c
		President, Chief Executive Officer and Director

KNOW ALL PERSONS BY THESE PRESENTS, that each person whose signature appears below hereby severally constitutes and appoints Angelos M. Stergiou and John T. Burns, and each of them singly, as such person's true and lawful attorneys-in-fact and agents, with full power of substitution and resubstitution, for such person and in such person's name, place and stead, in any and all capacities, to sign any and all amendments (including, without limitation, post-effective amendments) to this registration statement (or any registration statement for the same offering that is to be effective upon filing pursuant to Rule 462(b) under the Securities Act of 1933), and to file the same, with all exhibits thereto, and all documents in connection therewith, with the Securities and Exchange Commission, granting unto each said attorney-in-fact and agent, full power and authority to do and perform each and every act and thing requisite and necessary to be done in and about the premises, as fully to all intents and purposes as such person might or could do in person, hereby ratifying and confirming all that any said attorney-in-fact and agent, or any substitute or substitutes of any of them, may lawfully do or cause to be done by virtue hereof.

Pursuant to the requirements of the Securities Act of 1933, this registration statement has been signed by the following persons in the capacities and on the dates indicated.

Signature		Title		Date
/s/ Angelos M. Stergiou		President, Chief Executive Officer and Director		October 10, 2025
Angelos M. Stergiou, M.D., ScD h.c.		(Principal Executive Officer)
/s/ John T. Burns		Senior Vice President, Chief Financial Officer		October 10, 2025
John T. Burns		(Principal Financial and Accounting Officer)
/s/ John Varian		Chair of the Board of Directors		October 10, 2025
John Varian
/s/ David L. Scheinberg		Director		October 10, 2025
David L. Scheinberg, M.D., Ph.D.
/s/ Robert Van Nostrand		Director		October 10, 2025
Robert Van Nostrand
/s/ Jane Wasman		Director		October 10, 2025
Jane Wasman
/s/ Katherine Bach Kalin		Director		October 10, 2025
Katherine Bach Kalin