The 22-year-old, who stars in TV hit Buffy The Vampire Slayer, came top of the readers' poll in FHM magazine, beating British names such as singer Louise Nurding and Big Breakfast presenter Kelly Brook.
Buffy The Vampire Slayer - shown in the UK on BBC Two - has become one of the most popular teenage TV shows world-wide. Gellar has built up a cult following as a high school student with supernatural powers to hunt down and dispatch vampires.
An agent spotted her at the age of four while she was eating in a restaurant, and she has also starred in the films Scream 2 and I Know What You Did Last Summer.
The poll reflects her growing profile outside the US - she came in at 81st in the same poll last year.
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She told the magazine: "It's a real honour and I'm flattered. Obviously, the FHM readers don't see me first thing in the morning before a team of people come to fix my hair and make-up."
Other big names in the list include Entrapment star Catherine Zeta Jones, who comes in at sixth.
Children's TV presenter Gail Porter, who recently posed nude for the magazine, registered at eighth.
Meanwhile, Robbie Williams beat Hollywood's finest to come top of a parallel poll of Company magazine readers.
The triple Brit-award winner beat Brad Pitt, George Clooney and Will Smith to be named the sexiest man of the millennium.
Kelly Brook's Big Breakfast co-host, Johnny Vaughan, rated fifth, while Liverpool footballer Michael Owen comes in sixth.
Friends actors Matt Le Blanc and Matthew Perry registered at sixth and 10th, helping ensure all six stars of the show appear in both polls.
For Company Magazine's top 100 men, click here.
For FHM's top 100 women, click here.
FHM Online
Buffy The Vampire Slayer (BBC Online)
Official Buffy site
Robbie Williams
Friends
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