<PAGE>
As filed pursuant to Rule 424(b)(3)
Registration No. 333-78959
PROSPECTUS SUPPLEMENT DATED NOVEMBER 18, 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
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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>
Castle Convertible Fund, Inc. .................................................... $250,000 *% 3,030
MainStay VP Convertible Portfolio............................................... 500,000 * 6,060
Pebble Capital Inc.............................................................. 2,414,000 * 29,260
Putnam Convertible Income-Growth Trust.......................................... 355,000 * 4,303
Putnam Convertible Opportunities and Income Trust............................... 43,000 * 521
</TABLE>
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* less than one percent
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>
Boulder Capital Inc............................................................. $6,086,000 2.4% 73,769
Boulder II Limited.............................................................. 3,700,000 1.5 44,848
Museum of Fine Arts, Boston..................................................... 15,000 * 181
New Hampshire Retirement System................................................. 91,000 * 1,103
Onex Industrial Partners........................................................ 6,600,000 2.6 80,000
Parker-Hannifin Corporation..................................................... 19,000 * 230
ProMutual....................................................................... 56,000 * 678
Putnam Balanced Retirement Fund................................................. 42,000 * 509
State of Connecticut Combined Investment Funds.................................. 1,045,000 * 12,667
University of Rochester......................................................... 14,000 * 169
</TABLE>
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* less than one percent
