Worldwide recognition (1997–2003)
After reading the script for Joss Whedon's television series Buffy the Vampire Slayer, which follows Buffy Summers, a teenager burdened with the responsibility of fighting occult foes and supernatural occurrences, Gellar screen tested for the role of Cordelia Chase. Whedon then asked her to come back in and audition for the title role. The show premiered in March 1997, to widespread critical and popular acclaim. Gellar's Buffy, created to subvert the stereotypical female horror movie victim, was described by Entertainment Weekly as one of the 100 greatest female characters in American television. Buffy ran for seven seasons and 144 episodes, and during its broadcast, earned Gellar four Teen Choice Awards, the Saturn Award for Best Genre Television Actress, and a Golden Globe Award nomination for Best Actress – Television Series Drama. She sang during the series' musical episode "Once More, with Feeling", which spawned an original cast album, released in 2002.
During the early airing of Buffy the Vampire Slayer, Gellar made her first major film appearances in two successful slasher films. In I Know What You Did Last Summer (1997), she took on the role of ill-fated beauty queen Helen Shivers. Washington Post found the cast to be "solid", in what San Francisco Chronicle described as a "competent but uninspired" film. Budgeted at US$17 million, the film made US$125 million globally. For her part, Gellar earned a Blockbuster Entertainment Award for Favorite Supporting Actress – Horror and a MTV Movie Award nomination for Best Breakthrough Performance. In Scream 2 (1997), Gellar played a likewise ill-fated vain character, this time that of Sorority sister Cici Cooper. She filmed her scenes in between shots of Buffy and had only recently finished work on I Know What You Did Last Summer. Despite the hectic scheduling, she agreed to perform in Scream 2 without having read the script, on the basis of the success of the first film. Scream 2 grossed over US$172 million worldwide.
In January 1998, Gellar hosted Saturday Night Live for the first time. She returned as a host in May 1999 and October 2002, was a participating audience member in its 25th anniversary special, and made two cameo appearances in May 2000, including one in which she introduced Britney Spears' performance of "Don't Let Me Be the Last to Know". In 1998, she also provided her voice for the Gwendy Doll in Small Soldiers, and for the character of Marie in the King of the Hill episode "And They Call It Bobby Love". In 1999, Gellar had a non-speaking cameo as a girl sitting in the high school cafeteria in the sleeper hit She's All That, and obtained her first top-billing film role, as a struggling restaurant owner, in the romantic comedy Simply Irresistible. The film received negative reviews and flopped at the box office, but Roger Ebert found her to be "lovely" in what he described as an "old-fashioned" comedy.
In Roger Kumble's Cruel Intentions (1999), a modern-day retelling of Les Liaisons dangereuses, Gellar portrayed Kathryn Merteuil, a cocaine addict with an appetite for manipulating people. In his review for the film, Ebert felt that she is "effective as a bright girl who knows exactly how to use her act as a tramp", and in an interview with Chicago Tribune, Kumble described her as "the most professional actor I ever worked with". The film was a hit at the box office, grossing US$75 million worldwide, and went on to become a cult classic. Gellar and co-star Selma Blair obtained the Best Kiss award at the 2000 MTV Movie Awards. In Angel, a spin-off series of Buffy the Vampire Slayer, Gellar reprised her titular role for a three-episode arc, starting in 1999.
In 2000, Gellar appeared as a film studio executive in the HBO series Sex and the City episode "Escape from New York". Her next film, James Toback's independent drama Harvard Man (2001), in which she starred as the "sharp and shrewd" daughter of a mobster, helped her shed her good girl image, along with Cruel Intentions, according to Peter Travers of Rolling Stone. In 2002, Gellar portrayed Daphne Blake in the live action–comedy Scooby-Doo. For the production, she trained with a Hong Kong wire team, and commuted between Queensland and California every two weeks due to her simultaneous commitment to Buffy. Despite negative reviews, A. O. Scott of The New York Times felt that her performance added "a snarl of Powerpuff feminism to her character's ditzy stereotype", and with a global gross of US$275 million, Scooby-Doo emerged as Gellar's most widely seen film to date. Her role earned her the Teen Choice Award for Choice Movie Actress – Comedy. With Jack Black, she hosted the 2002 MTV Movie Awards, which attracted 7.1 million viewers on its June 6 broadcast, achieving the show's highest rating ever at the time.
During her growing film career, Gellar continued to work on Buffy the Vampire Slayer, but she decided to leave the series after the seventh season. When asked why, she explained, "This isn't about leaving for a career in movies, or in theater –it's more of a personal decision. I need a rest." In her feature in Esquire magazine, Gellar expressed her pride for her work on Buffy, "I truly believe that it is one of the greatest shows of all time and it will go down in history as that. And I don't feel that that is a cocky statement. We changed the way that people looked at television."