A string quintet is a musical composition for five string players. As an extension to the string quartet (two violins, a viola, and a cello), a string quintet includes a fifth string instrument, usually a second viola (a so-called "viola quintet") or a second cello (a "cello quintet"), or occasionally a double bass.

Notable examples of classic "viola quintets", in four movement form include those of Wolfgang Amadeus Mozart. Other examples were written by composers including Johannes Brahms and Felix Mendelssohn.

A famous "cello quintet" is Franz Schubert's Quintet in C major. Antonín Dvořák's Quintet Op. 77 uses a double bass, and Mozart's famous Eine kleine Nachtmusik may be performed with this instrumentation (the double bass being optional).

Alternative additions include clarinet or piano (see clarinet quintet, piano quintet); and other closely related chamber music genres include the string quartet (much more common), the string trio, and the string sextet. A more unusual form of string quintet is the violin quintet composed of 3 violins, a viola and a cello (thus a string quartet with an additional violin).

The term string quintet may refer to a group of five players that performs such works. The ensemble was standard in 17th century Italy and can be seen as early as 1607 in Claudio Monteverdi's opera, L'Orfeo.