In the Court of the Crimson King (subtitled An Observation by King Crimson) is the debut studio album by English progressive rock band King Crimson, released on 10 October 1969, by Island Records. The album is considered one of the earliest and most influential of the progressive rock genre, with the band combining musical influences that rock music was founded upon with elements of jazz, classical, and symphonic music.

Widely regarded as one of the greatest progressive rock albums of all time, In the Court of the Crimson King reached number five on the UK Albums Chart and number 28 on the US Billboard 200, where it was certified Gold by the Recording Industry Association of America (RIAA). In 2015, the album was ranked number 2 on Rolling Stone's list of the "50 Greatest Prog Rock Albums of All Time", behind 1973's The Dark Side of the Moon by Pink Floyd.