Dream Theater is an American progressive metal band formed in 1985 in Boston, Massachusetts. The band comprises John Petrucci (guitar), John Myung (bass), Mike Portnoy (drums), James LaBrie (vocals) and Jordan Rudess (keyboards).

Dream Theater was formed under the name Majesty by Petrucci, Myung and Portnoy — all natives of Long Island, New York — while they attended Berklee College of Music. They dropped out to concentrate on the band. Petrucci and Myung have been the only two constant members. Portnoy had remained until 2010, when he was replaced by Mike Mangini after deciding to leave to pursue other musical projects, before rejoining Dream Theater in October 2023. After a brief stint with Chris Collins, followed by Charlie Dominici (who was dismissed from Dream Theater shortly after the release of their 1989 debut album When Dream and Day Unite), LaBrie was hired as the band's singer in 1991. Dream Theater's first keyboardist, Kevin Moore, left the band after three albums and was replaced by Derek Sherinian in 1995 after a period of touring. The band recorded one album with Sherinian, who was replaced by current keyboardist Jordan Rudess in 1999.

Dream Theater has released fifteen studio albums. The band's highest-selling release is their second album Images and Words (1992), which reached No. 61 on the Billboard 200 chart. The albums Awake (1994) and Six Degrees of Inner Turbulence (2002) also entered the charts at No. 32 and No. 46 respectively, and each received critical acclaim. Their fifth album, Metropolis Pt. 2: Scenes from a Memory (1999), was ranked number 95 on the October 2006 issue of Guitar World magazine's list of the greatest 100 guitar albums of all time. It was also ranked as the 15th Greatest Concept Album in March 2003 by Classic Rock magazine.

By 2018, Dream Theater had sold over 12 million records worldwide and they have received three Grammy Award nominations (including one win in 2022). Along with Queensrÿche and Fates Warning, the band has been referred to as one of the "big three" of the progressive metal genre, responsible for its development and popularization.