Amnesiac is the fifth studio album by the English rock band Radiohead, released on 30 May 2001 by EMI. It was recorded with the producer Nigel Godrich in the same sessions as Radiohead's previous album Kid A (2000). Radiohead split the work in two as they felt it was too dense for a double album. As with Kid A, Amnesiac incorporates influences from electronic music, 20th-century classical music, jazz and krautrock. The final track, "Life in a Glasshouse", is a collaboration with the jazz trumpeter Humphrey Lyttelton and his band.

After having released no singles for Kid A, Radiohead promoted Amnesiac with the singles "Pyramid Song" and "Knives Out", accompanied by music videos. Videos were also made for "Pulk/Pull Revolving Doors" and "Like Spinning Plates", as well as "I Might Be Wrong", which was released as a promotional single. In June 2001, Radiohead began the Amnesiac tour, incorporating their first North American tour in three years and a homecoming performance in Oxford.

Amnesiac debuted at number one on the UK Albums Chart and number two on the US Billboard 200. By October 2008, it had sold over 900,000 copies worldwide. It is certified platinum in the UK, the US and Canada, and gold in Japan. It received positive reviews, though some critics felt it was too experimental or less cohesive than Kid A, or saw it as a collection of Kid A outtakes.

Amnesiac was named one of the year's best albums by numerous publications. It was nominated for the Mercury Prize and several Grammy Awards, winning for Best Recording Package for the special edition. "Pyramid Song" was named one of the best tracks of the decade by Rolling Stone, NME and Pitchfork, and Rolling Stone ranked Amnesiac number 320 in their 2012 list of the "500 Greatest Albums of All Time". Kid A Mnesia, an anniversary reissue compiling Kid A, Amnesiac and previously unreleased material, was released in 2021.

Radiohead and their producer, Nigel Godrich, recorded Amnesiac during the same sessions as their previous album, Kid A, released in October 2000. The sessions took place from January 1999 to mid-2000 in Guillaume Studios in Paris, Medley Studios in Copenhagen, and Radiohead's newly built studio in Oxfordshire. The drummer, Philip Selway, said the sessions had "two frames of mind ... a tension between our old approach of all being in a room playing together and the other extreme of manufacturing music in the studio. I think Amnesiac comes out stronger in the band-arrangement way."

The sessions drew influence from electronic music, 20th-century classical music, jazz and krautrock, using synthesisers, ondes Martenot, drum machines, strings and brass. The strings, arranged by the guitarist Jonny Greenwood, were performed by the Orchestra of St John's and recorded in Dorchester Abbey, a 12th-century church close to Radiohead's studio.

Radiohead considered releasing the work as a double album, but felt it was too dense. The singer, Thom Yorke, said Radiohead split it into two albums because "they cancel each other out as overall finished things" and came from "two different places". He felt Amnesiac offered a "different take" on Kid A and "a form of explanation". The band members stressed that they saw Amnesiac not as a collection of Kid A B-sides or outtakes but an album in its own right. Yorke said the title was inspired by a Gnostic belief that the trauma of birth erases memories of past lives, an idea he found fascinating.

"Pulk/Pull Revolving Doors" began as an attempt to record another song, "True Love Waits". It features keyboard loops recorded during the OK Computer sessions; Radiohead disabled the erase heads on the tape recorders so that the tape repeatedly recorded over itself, creating a "ghostly" tape loop, and manipulated the results in Pro Tools. Deciding that the arrangement did not fit "True Love Waits", Radiohead used it to create a new track. Yorke added a spoken vocal and used the pitch-correcting software Auto-Tune to process it into melody. According to Yorke, Auto-Tune "desperately tries to search for the music in your speech, and produces notes at random. If you've assigned it a key, you've got music." The "True Love Waits" version of "Pulk/Pull Revolving Doors" was eventually released on the 2021 compilation Kid A Mnesia.