Prospectus Certification by Investment Company (Form 497J)

May 5, 2025

VIA ELECTRONIC TRANSMISSION

U.S. Securities and Exchange Commission

100 F Street N.E.

Washington, D.C. 20549

Dear Sir or Madam:

Pursuant to Rule 497(j) under the Securities Act of 1933 (the "Securities Act"), the undersigned hereby certifies on behalf of William Blair Funds (the "Trust") that the form of Prospectus for all series of the Trust and Statement of Additional Information included in Post-Effective Amendment No. 155 to the Trust's Registration Statement on Form N-1A ("PEA 155") that would have been filed by the Trust pursuant to Rule 497(c) under the Securities Act would not have differed from what was contained in PEA 155. I hereby further certify that PEA 155 was filed electronically with the Commission on April 30, 2025 pursuant to Rule 485(b) under the Securities Act.

No fees are required in connection with this filing. Please call me at (312) 364-8209 if you have any questions.