QuickLinks

As filed with the Securities and Exchange Commission on February 11, 2000

Registration No. 333-94537

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

POST-EFFECTIVE
AMENDMENT NO. 1
TO
FORM S-4
REGISTRATION STATEMENT
UNDER
THE SECURITIES ACT OF 1933

YAHOO! INC.
(Exact Name of Registrant as Specified in its Charter)

3420 Central Expressway
Santa Clara, California 95051
(408) 731-3300
(Address, Including Zip Code, and Telephone Number, Including Area Code, of
Registrant(1)s Principal Executive Offices)

Gary Valenzuela
Senior Vice President, Finance and Administration, and Chief Financial Officer
Yahoo! Inc.
3420 Central Expressway
Santa Clara, California 95051
(408) 731-3300
(Name, Address, Including Zip Code, and Telephone Number, Including Area Code, of Agent for Service)

With Copies to:
Joshua L. Green
Steven J. Tonsfeldt
Keith A. Miller
Venture Law Group
A Professional Corporation
2800 Sand Hill Road
Menlo Park, California 94025
(650) 854-4488

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

   If the securities being registered on this form are being offered in connection with the formation of a holding company and there is compliance with general Instruction G, check the following box. / /

   If this form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act of 1933 (the Securities Act(2)), 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(d) 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 any of the securities being registered in this Form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act, check the following box. /x/

CALCULATION OF REGISTRATION FEE

(1)

The shares of common stock set forth in the Calculation of Registration Fee Table, and which may be offered pursuant to this Registration Statement, includes, pursuant to Rule 416 of the Securities Act of 1933, as amended, such additional number of shares of the Registrant's common stock that may become issuable as a result of any stock splits, stock dividends or similar event.

(2)

Estimated solely for the purpose of computing the amount of the registration fee, based on the average of the high and low prices for the Company's common stock as reported on the Nasdaq National Market on January 7, 2000 in accordance with Rule 457 under the Securities Act of 1933.

   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 which specifically states that this registration statement shall thereafter become effective in accordance with Section 8(a) of the Securities Act or until the registration statement shall become effective on such date as the Securities and Exchange Commission, acting pursuant to said Section 8(a), may determine.

    In accordance with Rule 416(b) promulgated under the Securities Act of 1933, as amended (the "Securities Act"), the number of shares of Common Stock registered for sale under the Securities Act by this Registration Statement on Form S-4 has been deemed to be increased to include the shares of Common Stock issued in connection with the two-for-one stock split effected as a stock dividend on February 11, 2000 (the "Stock Split"), to the extent issued with respect to shares designated by such registration statement but unsold as of the date of the Stock Split.

SIGNATURES

    Pursuant to the requirements of the Securities Act of 1933, as amended, Yahoo! Inc. has duly caused this amendment to the registration statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Santa Clara, State of California, on February 11, 2000.

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

QuickLinks

SIGNATURES