Form 485BXT WILLIAM BLAIR FUNDS

As filed with the Securities and Exchange Commission on or about February 25, 2026

Registration No. 033-17463 and 811-05344

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM N-1A

REGISTRATION STATEMENT

UNDER

and

REGISTRATION STATEMENT

UNDER

WILLIAM BLAIR FUNDS

(Exact Name of Registrant as Specified in Charter)

150 North Riverside Plaza Chicago, Illinois 60606

(Address of Principal Executive Offices, including Zip Code)

Registrant’s Telephone Number, Including Area Code: (312) 364-8000

It is proposed that this filing will become effective (check appropriate box)

If appropriate, check the following box:

Approximate Date of Proposed Public Offering: As soon as practicable after the effective date of the registration statement

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933 and the Investment Company Act of 1940, the Registrant certifies that it meets all of the requirements for effectiveness of this registration statement under Rule 485(b) under the Securities Act of 1933 and that it has duly caused this Post-Effective Amendment No. 158 to the registration statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Chicago, and State of Illinois, on the 25th day of February, 2026.

Pursuant to the requirements of the Securities Act of 1933, this Post-Effective Amendment No. 158 to the registration statement has been signed below by the following persons in the capacity indicated on the 25th day of February, 2026.

* Lisa Rusch signs this document pursuant to powers of attorney filed as an exhibit to Post-Effective Amendment No. 156.