Bosworth's first film role came after an open casting call in New York for the supporting part of Judith in the 1998 film, The Horse Whisperer. The film's producers needed someone who was already an experienced horse rider, leading to Bosworth's successful audition for the role. In 2000, she starred in the television series drama Young Americans, in which she played Bella Banks, a well-adjusted small town girl working at a gas station near an elite prep school. The series, however, was cancelled. That same year, she had a small part in the film Remember the Titans. In 2001, Bosworth moved to Los Angeles in hopes of obtaining easier access to auditions and better film parts. Her first leading role was in 2002's surfing movie Blue Crush, which she prepared for by working out with two separate trainers six hours a day for months in order to add fifteen pounds of muscle to her frame. The film received positive reviews and grossed $40 million at the United States box office, allowing Bosworth wider exposure to a mainstream audience.

After Blue Crush, Bosworth took on roles in the indie films The Rules of Attraction and a darker turn in Wonderland opposite Val Kilmer, where she played the teenage girlfriend of porn star John Holmes. Between 2002 and 2005, Bosworth appeared in a number of widely released films. She starred as actress Sandra Dee in Beyond the Sea and played the lead role in Win a Date with Tad Hamilton!, opposite Topher Grace. Bosworth also portrayed Chali, a Hare Krishna, in a film adaptation of Myla Goldberg's acclaimed novel Bee Season, about a dysfunctional Jewish family. She has appeared in several Revlon ads and ranked as #60 in FHM's "100 Sexiest Women in the World 2005" list. For two years running, she has placed on the Maxim Hot 100 List – number 38 in 2005 and number 8 in 2006.

After going through many screen tests and competing with other actresses for the role, she was cast as reporter Lois Lane in the highly anticipated adaptation to the Superman comics, Superman Returns. She starred along with her Beyond the Sea co-star Kevin Spacey as Lex Luthor, as well as newcomer Brandon Routh as Superman. The film was a box-office success and received mostly positive reviews. Bosworth's performance however, was not well-received among critics; Anthony Lane of The New Yorker, in his review of the film noted, "The new Lois Lane, Kate Bosworth, is not a patch on Margot Kidder, or, for that matter, on Teri Hatcher, in the TV series." San Francisco Chronicle film critic Mick LaSalle felt that Bosworth, at 22-years-old, was too young to portray Lois Lane, and the climax did not "match the potential of the tiring 154 minute long film". Although Bosworth's performance was ambiguously received, she herself loved the experience of working on the movie. As she told Teen Vogue in August 2006, "You know how you have an experience, a time in your life when you feel you've come into your own? When you grow up a bit, and think, Now I get it? That's how I feel. I feel a little bit more complete." Bosworth starred in the psychological drama called The Girl in the Park with Sigourney Weaver, Alessandro Nivola and Keri Russell. She plays Louise, a troubled young girl taken under Weaver's character's wing. The Girl in the Park was written and directed by Pulitzer Prize winner David Auburn in his directorial debut.[24] The film premiered at the 2007 Toronto International Film Festival, it has since been picked up by The Weinstein Company.

Bosworth also filmed 21, an adaptation of the book Bringing Down the House, in early 2007 in Boston, Massachusetts and Las Vegas, Nevada. This film reunites her with co-star/director Kevin Spacey and director Robert Luketic. She recently wrapped the epic Laundry Warrior in New Zealand alongside Korean star Jang Dong-gun and Geoffrey Rush. She has also optioned the film rights to Catherine Hanrahan's novel Lost Girls and Love Hotels, about a young woman looking to escape in Tokyo's seedy nightlife. She will produce the project alongside filmmaker Nadia Connors. She has said about her roles, "I just don't do comfort zones." In January 2008, Bosworth was named the new face for Calvin Klein Jeans and will be featured in the Calvin Klein Jeans campaign. She also takes on the role of spokesperson for American luxury bag brand Coach in Asia.

• Lost Girls and Love Hotels (2010) (pre-production) .... Margaret
• The Warrior's Way (2009) (completed) .... Lynne
  ... aka The Warrior (USA: informal alternative title)
  
• 21 (2008) .... Jill Taylor
  ... aka 21 - The Movie (USA: promotional title)
• The Girl in the Park (2007) .... Louise
• Superman Returns (2006) (VG) (voice) .... Lois Lane
• Superman Returns (2006) .... Lois Lane
  ... aka Superman Returns: An IMAX 3D Experience (USA: IMAX version)
• Bee Season (2005) .... Chali
• Beyond the Sea (2004) .... Sandra Dee
  ... aka Beyond the Sea - Musik war sein Leben (Germany)
• Win a Date with Tad Hamilton! (2004) .... Rosalee Futch
• Advantage Hart (2003) .... Trinity Montage
• Wonderland (2003) .... Dawn Schiller
• The Rules of Attraction (2002) .... Kelly
  ... aka Die Regeln des Spiels (Germany)
• Blue Crush (2002) .... Anne Marie Chadwick
  ... aka Blue Crush (Germany)
• Remember the Titans (2000) .... Emma Hoyt
• "Young Americans" .... Bella Banks (8 episodes, 2000)
       - Will Bella Scout Her Mom? (2000) TV episode .... Bella Banks
       - Free Will (2000) TV episode .... Bella Banks
       - Gone (2000) TV episode .... Bella Banks
       - Winning Isn't Everything (2000) TV episode .... Bella Banks
       - Cinderbella (2000) TV episode .... Bella Banks
        (3 more)
• The Newcomers (2000) .... Courtney Docherty