<PAGE>
As filed pursuant to Rule 424(b)(3)
Registration No. 333-78959
PROSPECTUS SUPPLEMENT DATED FEBRUARY 16, 2000
TO PROSPECTUS DATED AUGUST 18, 1999, AS SUPPLEMENTED OCTOBER 20, 1999,
NOVEMBER 1, 1999, NOVEMBER 18, 1999, DECEMBER 6, 1999 AND
JANUARY 10, 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
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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 SHARES OF
AMOUNT OF PERCENTAGE OF COMMON STOCK
NOTES THAT NOTES THAT MAY BE
NAME OF SELLING SECURITYHOLDER MAY BE SOLD OUTSTANDING SOLD(1)
- ------------------------------ ----------- ----------- -------
<S> <C> <C> <C>
Bankers Trust Trustee for Chrysler Corp Emp #1
Pension Plan dtd 4/1/89........................... $2,183,000 * % 52,921
Franklin & Marshall College......................... 167,000 * 4,048
</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 SHARES OF
AMOUNT OF PERCENTAGE OF COMMON STOCK
NOTES THAT NOTES THAT MAY BE
NAME OF SELLING SECURITYHOLDER MAY BE SOLD OUTSTANDING SOLD(1)
- ------------------------------ ----------- ----------- -------
<S> <C> <C> <C>
Boston Museum of Fine Arts.......................... $ 85,000 * % 2,060
Chase Manhattan, N.A. Trustee for IBM Retirement
Plan dtd 12/18/45................................. 2,838,000 1.1 68,800
San Diego City Retirement........................... 599,000 * 14,521
San Diego County Convertible........................ 1,623,000 * 39,345
State Street Bank Custodian for
GE Pension Trust.................................. 1,162,000 * 28,169
</TABLE>
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* 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.
