4chan is an English-language (English language) imageboard website. Launched on October 1, 2003, its boards are primarily used for the posting of pictures and discussion of manga and anime. Users generally post anonymously and the site has been linked to Internet subcultures and activism, most notably Project Chanology.

4chan users have been responsible for the formation or popularization of Internet memes such as lolcats, Rickrolling, "Chocolate Rain", "Pedobear (#"Pedobear")", and many others. The sites random board is by far its most popular and notorious. Known as "/b/", there are very minimal rules on posted content. Gawker.com once claimed in jest that "reading /b/ will melt your brain".

The sites Anonymous (Anonymous (group)) community and culture has often provoked media attention. For planners (media planner), this enterprise is "further proof that creativity is everywhere and new media is less accessible" to advertisement agencies (advertising agency). Journalists looked at how an Internet destination was hijacked for a prank, so that images of Rick Astley (rickroll) appeared instead of the page that was searched for; the coordination of attacks against other websites and Internet users; and covered the reaction to threats of violence that have been posted on the site. The Guardian once summarised the 4chan community as "lunatic, juvenile... brilliant, ridiculous and alarming."