Jack Six (July 26, 1930 – March 14, 2015) was an American jazz double-bassist and composer.

Six was born in Danville, Illinois, and initially learned trumpet as a teenager before switching to bass. He studied at Juilliard in 1955–1956, then played in several big bands, including the Tommy Dorsey band after Dorsey's death (under Warren Covington's direction) and the bands of Claude Thornhill and Woody Herman. In the first half of the 1960s he played with Don Elliott, Jimmy Raney, Kenny Davern, The Dukes of Dixieland, and Herbie Mann; he continued working with Davern alongside Dick Wellstood as the house band for the Ferryboat, a club in Brielle, New Jersey. He became a member of Dave Brubeck's ensemble in 1968, remaining with Brubeck until 1974, and also played with Tal Farlow during this time. In the 1970s he worked with Illinois Jacquet and Jay McShann, among others. He directed musical events at a casino in Atlantic City in the early 1980s and recorded with Susannah McCorkle and Jack Reilly; later that decade he returned to Brubeck's ensemble, remaining with him until he retired in the 1990s.