Alt-J (stylised as alt-J, or ∆) is an English indie rock band formed in 2007 in Leeds, by Joe Newman (guitar/lead vocals), Thom Sonny Green (drums), Gus Unger-Hamilton (keyboards/vocals) and Gwil Sainsbury (guitar/bass).

The band's debut album An Awesome Wave was released in May 2012 in Europe, and in September 2012 in the United States, and won the 2012 British Mercury Prize. Gwil Sainsbury amicably departed the band in early 2014. Their second album, This Is All Yours, was released on 22 September 2014 and went straight to UK number one. As a replacement for Sainsbury, Cameron Knight became a supporting member for alt-J's live shows, playing guitar, bass and sampler. In 2017, the band released their third studio album, Relaxer, and are currently playing as a trio.