4chan is an anonymous English-language imageboard website. Launched by Christopher "moot" Poole in October 2003, the site hosts boards dedicated to a wide variety of topics, from video games and television to literature, cooking, weapons, music, history, technology, anime, physical fitness, politics, and sports, among others. Registration is not available, except for staff, and users typically post anonymously. As of 2022, 4chan receives more than 22 million unique monthly visitors, of whom approximately half are from the United States.

4chan was created as an unofficial English-language counterpart to the Japanese imageboard Futaba Channel, also known as 2chan, and its first boards were originally used for posting images and discussion related to anime. The site has been described as a hub of Internet subculture, its community being influential in the formation and popularization of prominent Internet memes, such as lolcats, Rickrolling, rage comics, wojaks, Pepe the Frog, as well as hacktivist and political movements, such as Anonymous and the alt-right.

4chan has often been the subject of media attention as a source of controversies, including the coordination of pranks and harassment against websites and Internet users, and the posting of illegal and offensive content as a result of its lax censorship and moderation policies. In 2008, The Guardian summarized the 4chan community as "lunatic, juvenile [...] brilliant, ridiculous and alarming".

On April 14, 2025, the site was taken down following a hack. On another website, soyjak.party, information purporting to be from the hack was released, claiming "admin" level access, a lack of updates to the site since 2016, and identities of admins, among other claims. Following the attacks, 4chan owner and administrator Hiroyuki Nishimura released statements to their official Twitter accounts confirming the site would return at a later date. The site returned on April 25.