William Parker (born January 10, 1952) is an American free jazz double bassist. Beginning in the 1980s, Parker played with Cecil Taylor for over a decade, and he has led the Little Huey Creative Music Orchestra since 1981. The Village Voice named him "the most consistently brilliant free jazz bassist of all time" and DownBeat has called him "one of the most adventurous and prolific bandleaders in jazz".

Active since the early 1970s, Parker first came to public attention playing with pianist Cecil Taylor in the 1980s. His work as leader came to greater prominence in the 1990s with groups such as the Little Huey Creative Music Orchestra and In Order to Survive. Parker's "breakout" albums were released in the early 2000s, first with the William Parker Quartet (with saxophonist Rob Brown, drummer Hamid Drake, and trumpeter Lewis Barnes): O'Neal's Porch was included in Best of 2001 lists in The New York Times, DownBeat, and the Jazz Journalists Association; in 2002, Raining on the Moon, featuring guest Leena Conquest, received rave reviews in publications including Pitchfork. Increasing prominence throughout the 2000s also led to a revisiting of his back catalogue, with the release of a number of early recordings. Parker is a prominent musician in the New York City experimental jazz scene where he leads a number of groups, and has performed at festivals around the world.

Bass has been his primary instrument for the duration of his career, but he also plays trumpet, tuba, bamboo flutes, shakuhachi, flute, double reeds, the West African kora, gembri, and donso ngoni, an instrument first introduced to him by Don Cherry. Methodically, Parker frequently plays "arco".