The bass saxophone is the third lowest member of the saxophone family—larger and lower than the more common baritone saxophone. It was likely the first type of saxophone built by Adolphe Sax, as first observed by Berlioz in 1842. It is a transposing instrument pitched in B♭, an octave below the tenor saxophone and a perfect fourth below the baritone saxophone. A bass saxophone in C, intended for orchestral use, was included in Adolphe Sax's patent, but few known examples were built. The bass saxophone is not a commonly used instrument, but it is heard on some 1920s jazz recordings, in free jazz, in saxophone choirs and sextets, and occasionally in concert bands and rock music.

Music for bass saxophone is written in treble clef, just as for the other saxophones, with the pitches sounding two octaves and a major second lower than written. As with most other members of the saxophone family, the lowest written note is the B♭ below the staff—in the bass's case, sounding as a concert A♭₁.

The bass saxophone was one of only two working prototypes completed before Sax's 1846 patent, the other being an "E♭ tenor" (now the baritone). It was intended to replace the ophicleide, from which it was derived, and had a similar upright form. The bass was later revised by Sax into its current S-shaped form, first appearing in 1850 in a sales brochure.

The German wind instrument maker Benedikt Eppelsheim and the Brazilian saxophone maker J'Élle Stainer have both made bass saxophones with an additional low A key, similar to the low A key on the baritone saxophone. On the bass, this extends the range down a semitone to concert G₁ (~49 Hz). Most basses made before the 1980s were keyed to high E♭, but most new models are keyed to high F♯.

The bass saxophone enjoyed some popularity in jazz combos and dance bands between World War I and World War II, primarily providing bass line, although bass sax players occasionally took melodic solos. Notable players of this era include Billy Fowler, Coleman Hawkins, Otto Hardwicke (of the Duke Ellington orchestra), Adrian Rollini (who was a pioneer of bass sax solos in the 1920s and 30s), Min Leibrook, Spencer Clark, Charlie Ventura, and Vern Brown of the Six Brown Brothers. Sheet music of the period shows many bands photographed with a bass sax. The bass sax virtually disappeared in the 1930s, possibly due to its size, mechanical complexity, and high price. The invention of the electric bass guitar in the 1950s and its quick rise to popularity reduced demand for other bass instruments in popular music and other contemporary music.

American bandleader Boyd Raeburn (1913–1966) led an avant-garde big band in the 1940s and sometimes played the bass saxophone. In Britain, Oscar Rabin played it in his own band. Harry Gold, a member of Rabin's band, played bass saxophone in his own band, Pieces of Eight. American bandleader Stan Kenton's Mellophonium Band (1960–1963) featured fourteen brass players and used a saxophone section of one alto, two tenors, baritone, and bass on many Grammy winning compositions by Johnny Richards (with Joel Kaye doubling baritone and bass saxophones). The Lawrence Welk Band featured Bill Page soloing on bass saxophone on several broadcasts during the 1960s. Shorty Rogers's Swingin' Nutcracker (recorded for RCA Victor in 1960) featured a bass saxophone (played by Bill Hood) on four of the movements.

One notable bass saxophonist performing today in the 1920s–1930s style is Vince Giordano. Jazz players using the instrument in a more contemporary style include Roscoe Mitchell, Anthony Braxton, Peter Brötzmann, J. D. Parran, Hamiet Bluiett, James Carter, Stefan Zeniuk, Michael Marcus, Vinny Golia, Joseph Jarman, Brian Landrus, Urs Leimgruber, and Scott Robinson, although none of these players use it as their primary instrument.

Jan Garbarek plays a bass sax on the 1973 album Red Lanta.