28 Days Later is a 2002 British post-apocalyptic horror film directed by Danny Boyle and written by Alex Garland. It stars Cillian Murphy as a bicycle courier who awakens from a coma to discover the accidental release of a highly contagious, aggression-inducing virus has caused the breakdown of society. Naomie Harris, Christopher Eccleston, Megan Burns, and Brendan Gleeson appear in supporting roles.

Garland took inspiration from George A. Romero's Night of the Living Dead film series and John Wyndham's 1951 novel The Day of the Triffids. Filming took place in various locations in the United Kingdom in 2001. The crew filmed for brief periods during early mornings and temporarily closed streets to capture recognisable and typically busy areas when they were deserted. John Murphy composed an original soundtrack for the film, with other instrumental songs by Brian Eno, Godspeed You! Black Emperor and other artists also being featured.

28 Days Later was released on 1 November 2002 to critical acclaim and financial success. Grossing more than $82.7 million worldwide on its modest budget of $8 million, it became one of the most profitable horror films of 2002. Reviewers praised Boyle's direction, the cast's performances, Garland's screenplay, the atmosphere and soundtrack. Despite Boyle not considering it a zombie film, 28 Days Later is credited with reinvigorating the zombie genre of horror film and influencing a revival in the genre a decade after its release, with its fast-running infected and character-driven drama. Since its release, it has been featured in several "best-of" film lists and maintained a following, particularly amid the COVID-19 pandemic in the 2020s.

The film's success launched its titular film series, including 28 Weeks Later (2007), and 28 Years Later which is scheduled for release in 2025. Additional sequels are currently in development, while a graphic novel titled 28 Days Later: The Aftermath, and the 2009—2011 comic book series 28 Days Later also expanded the series into other media.