<PAGE>
As filed pursuant to Rule 424(b)(3)
Registration No. 333-78959
PROSPECTUS SUPPLEMENT DATED NOVEMBER 1, 1999
TO PROSPECTUS DATED AUGUST 18, 1999
DOUBLECLICK INC.
$250,000,000 4.75% Convertible Subordinated Notes due 2006
(Interest Payable March 15 and September 15)
--------------------
3,030,303 Shares of Common Stock
--------------------
The information contained in the table appearing under the heading "Selling
Securityholders" on pages 40 - 42 of the Prospectus is hereby amended to add the
following information:
<TABLE>
<CAPTION>
Principal
Amount of Percentage Shares of
Notes that of Notes Common Stock
Name of Selling Securityholder May be Sold Outstanding that May be Sold
- ------------------------------------------------------------------------------------------
<S> <C> <C> <C>
D. E. Shaw.................................. $ 30,000 * % 363
IBM Retirement Plan......................... 1,123,000 * 13,612
Morgan Stanley Dean Witter.................. 1,675,000 * 20,303
Credit Suisse First Boston Corporation...... 500,000 * 6,060
</TABLE>
The information contained in the table appearing under the heading "Selling
Securityholders" on pages 40 - 42 of the Prospectus with respect to the Selling
Securityholders named below is hereby deleted and restated with the following
information:
<TABLE>
<CAPTION>
Principal
Amount of Percentage Shares of
Notes that of Notes Common Stock
Name of Selling Securityholder May be Sold Outstanding that May be Sold
- -----------------------------------------------------------------------------------------------------------
<S> <C> <C> <C>
California Public Employees Retirement System................ 5,000,000 2.0 % 60,606
Chrysler Corporation Master Retirement Trust................. 880,000 * 10,666
Delta Air Line Master Trust.................................. 360,000 * 4,363
IBM Retirement Plan.......................................... 1,123,000 * 13,612
Motion Picture Industry Health Plan - Active Member Fund..... 105,000 * 1,272
Motion Picture Industry Health Plan - Retiree Member Fund.... 55,000 * 666
OCM Convertible Trust........................................ 465,000 * 5,636
OCM Covertible Limited Partnership........................... 25,000 * 303
Partner Reinsurance Company Ltd.............................. 105,000 * 1,272
State Employees' Retirement Fund of the State of Delaware.... 310,000 * 3,757
State of Connecticut Combined Investment Funds............... 105,000 * 1,272
Vanguard Convertible Securities Fund, Inc.................... 665,000 * 8,060
</TABLE>
