Barry John Guy (born 22 April 1947, in London) is an English composer and double bass player. His range of interests encompasses early music, contemporary composition, jazz and improvisation, and he has worked with a wide variety of orchestras in the UK and Europe. He studied at the Guildhall School of Music under Buxton Orr, and later taught there.

Guy came to the fore as an improvising bassist as a member of a trio with pianist Howard Riley and drummer Tony Oxley (Witherden, 1969). He also became an occasional member of John Stevens' ensembles in the 1960s and 1970s, including the Spontaneous Music Ensemble. In the early 1970s, he was a member of the influential free improvisation group Iskra 1903 with Derek Bailey and trombonist Paul Rutherford (a project revived in the late 1970s, with violinist Philipp Wachsmann replacing Bailey). He also formed a long-standing partnership with saxophonist Evan Parker, which led to a trio with drummer Paul Lytton which became one of the best-known and most widely travelled free-improvising groups of the 1980s and 1990s. He was briefly a member of the Michael Nyman Band in the 1980s, performing on the soundtrack of The Draughtsman's Contract.