The bass clarinet is a musical instrument of the clarinet family. Like the more common soprano B♭ clarinet, it is usually pitched in B♭ (meaning it is a transposing instrument on which a written C sounds as B♭), but it plays notes an octave below the soprano B♭ clarinet.
The bass clarinet has been regularly used in scoring for orchestra and concert band since the mid-19th century, becoming more common during the middle and latter part of the 20th century. A bass clarinet is not always called for in orchestra music, but is almost always called for in concert band music. In recent years, the bass clarinet has also seen a growing repertoire of solo literature including compositions for the instrument alone, or accompanied by piano, orchestra, or other ensemble. It is also used in clarinet choirs, marching bands, and in film scoring, and has played a persistent role in contemporary jazz. Someone who plays a bass clarinet is called a bass clarinettist or a bass clarinetist.