King Crimson are an English progressive rock band formed in London in 1968 by Robert Fripp, Michael Giles, Greg Lake, Ian McDonald and Peter Sinfield. Guitarist and leader Fripp has remained the only constant member throughout the band's long history. The band drew inspiration from a wide variety of music, incorporating elements of classical, jazz, folk, heavy metal, gamelan, blues, industrial, electronic, and experimental music. They also exerted a strong influence on the early 1970s progressive rock movement, including on contemporaries such as Yes and Genesis, and continue to inspire subsequent generations of artists across multiple genres. The band has earned a large cult following, especially in the 21st century.

The band's debut album, In the Court of the Crimson King (1969), remains their most commercially successful and influential release. The next two albums, In the Wake of Poseidon and Lizard (both 1970), were recorded during a period of instability in the band's line-up, before a stable group with Fripp, Sinfield, Mel Collins, Boz Burrell and Ian Wallace recorded Islands in 1971. In mid-1972, Fripp disbanded this line-up, recruited new members Bill Bruford (formerly of Yes), John Wetton (formerly of Family), David Cross and Jamie Muir, and changed the group's musical approach, drawing from European free improvisation and modernist classical music and developing ever more complex compositions. The band reached what some^[who?] saw as a creative peak on Larks' Tongues in Aspic (1973), Starless and Bible Black (1974), and Red (1974). Fripp disbanded the group at the end of 1974.

After seven years of inactivity, King Crimson was recreated in 1981 with Fripp, Bruford and new American members Adrian Belew and Tony Levin. Drawing influence from African music, gamelan, post-punk and New York minimalism, this band lasted three years, recording the albums Discipline (1981), Beat (1982) and Three of a Perfect Pair (1984). Following a decade-long hiatus, they reformed in 1994, adding Pat Mastelotto (formerly of Mr. Mister) and Trey Gunn for a sextet line-up Fripp called "The Double Trio". The double trio participated in another three-year cycle of activity that included the EP Vrooom (1994) and the album Thrak (1995). Fripp, Belew, Mastelotto and Gunn reunited in 2000 as a quartet, called "The Double Duo", releasing The Construkction of Light (2000) and The Power to Believe (2003). After another hiatus, the band reformed for a 2008 tour celebrating the 40th anniversary of their 1968 formation, with Gavin Harrison of Porcupine Tree being added, and Levin returning in place of Gunn.

Following another hiatus (2009–2012), during which Fripp was thought to be retired, King Crimson came together again in 2013, this time as a septet (and, later, octet) with an unusual three-drumkit frontline, and new singer and secondary guitarist Jakko Jakszyk. This version of King Crimson toured from 2014 to 2021. After the band's final show in 2021, Fripp commented that King Crimson had "moved from sound to silence". However, band members have made studio recordings of their parts for material written during the last phase of the band's live work as of July 2025, which may or may not become a new album according to band manager David Singleton.

In August 1967, brothers Michael and Peter Giles, drummer and singer/bassist respectively and pro musicians in working bands since their mid-teens in Dorset, England, advertised for a "singing organist" to join a group they were forming. Fellow Dorset musician Robert Fripp – a guitarist who neither played organ nor sang – responded, and Giles, Giles and Fripp was born. The trio recorded several quirky singles and one eclectic album, The Cheerful Insanity of Giles, Giles and Fripp. They hovered on the edge of success, and even made a television appearance, but were never able to make a commercial breakthrough.

Attempting to expand their sound, the three recruited Ian McDonald on keyboards and woodwinds. McDonald brought along two new participants: his then-girlfriend, former Fairport Convention singer Judy Dyble, whose brief tenure with the group ended when the two split, and lyricist, roadie, and art strategist Peter Sinfield, with whom he had been writing songs – a partnership initiated when McDonald had said to Sinfield (regarding his band Creation), "Peter, I have to tell you that your band is hopeless, but you write some great words. Would you like to get together on a couple of songs?" Fripp, meanwhile, saw Clouds at the Marquee Club in London, which spurred him to incorporate classically inspired melodies into his writing and utilise improvisation to find new ideas.

No longer interested in Peter Giles's more whimsical pop songs, Fripp recommended that his old friend, fellow guitarist and singer Greg Lake, could join to replace either Peter or Fripp himself. Peter Giles later called it one of Fripp's "cute political moves". According to Michael Giles, his brother had become disillusioned with the band's lack of success and departed before Fripp suggested Lake to fill Peter Giles's position as bassist and singer.

The first incarnation of King Crimson—Fripp, Michael Giles, Lake, McDonald and Sinfield—was formed on 30 November 1968 with rehearsals beginning on 13 January 1969. Sinfield coined the band's name in "a moment of pressured panic". Sinfield had already used the term "crimson king" in a set of lyrics before his involvement with Giles, Giles and Fripp. Sinfield insisted that the name did not refer to Beelzebub, prince of demons, and that a "crimson king" was any ruler during whose reign there were "societal rumblings" and "sort of the dark forces of the world". According to Fripp, King Crimson is a synonym for Beelzebub, which is an anglicised form of the Arabic phrase "B'il Sabab", meaning "the man with an aim", to which he related.