Rhythm section refers to the musicians whose primary jobs in a jazz or popular music band or ensemble is to establish the rhythm of a song or musical piece, often repeated riffs or ostinatos. It may also refer to the instruments of those musicians.

In theory, any instrument or instruments can provide a steady rhythm (see Jimmy Giuffre's clarinet-valve trombone-guitar trio of the late 1950s, for example), and "rhythm" instruments often take featured solos, especially in jazz.

Most commonly, however, are a drum set, and a bass guitar or double bass. A rhythm guitar is a common feature, and piano, organ, auxiliary percussion and/or other instruments may be used.

In some cases, the lower octaves of a piano, organ, or electronic keyboard may substitute for bass guitar. One of the best known examples of this was keyboardist Ray Manzarek of The Doors. Some jazz bands use tuba or other low-pitched instruments in place of the more common double bass.