A string quintet is a musical composition for five string players. As an extension to the string quartet, a string quintet includes a fifth string instrument, usually a second viola or a second cello, or occasionally a double bass.

The term string quartet refers to either a type of musical composition or a group of four people who play them. Many composers from the mid-18th century onwards wrote string quartets. The associated musical ensemble consists of two violinists, a violist, and a cellist. The double bass is almost never used in the ensemble mainly because it would sound too loud and heavy.