<PAGE>
As filed pursuant to Rule 424(b)(3) and (c)
Registration No. 333-78959
PROSPECTUS SUPPLEMENT DATED AUGUST 17, 2000
TO PROSPECTUS DATED AUGUST 18, 1999, AS SUPPLEMENTED OCTOBER 20, 1999,
NOVEMBER 1, 1999, NOVEMBER 18, 1999, DECEMBER 6, 1999,
JANUARY 10, 2000, FEBRUARY 11, 2000, MAY 8, 2000 AND JULY 31, 2000
DOUBLECLICK INC.
$250,000,000 4.75% Convertible Subordinated Notes due 2006
(Interest Payable March 15 and September 15)
--------------------
6,060,606 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 Shares of
Amount of Percentage Common Stock
Notes that of Notes that May be
Name of Selling Securityholder May be Sold Outstanding Sold (1)
-----------------------------------------------------------------------------------------------------
<S> <C> <C> <C>
Pacific Life Insurance Company............................ $500,000 * 12,121
</TABLE>
----------------------
* less than one percent
(1) Assumes a conversion price of $41.25 per share (adjusted to reflect the
2-for-1 split of common stock effected on January 10, 2000) and a cash
payment in lieu of any fractional interest.
