A string trio is a group of three string instruments or a piece written for such a group. The most usual such group in classical chamber music is made up of a violin, a viola and a cello.

The earliest string trio form consisted of two violins and cello, a grouping which had grown out of the baroque trio sonata. Joseph Haydn is thought to have been the first to have written for violin, viola and cello, and Ludwig van Beethoven and Franz Schubert also wrote music in that form, although they are not generally reckoned to be amongst their better pieces.

A number of composers wrote string trios throughout the 19th century, and in the 20th century, string trios by Arnold Schoenberg, Anton Webern and La Monte Young were produced.

Examples of more unusual string trio groups include Wolfgang Amadeus Mozart's trio for two violins and double bass, and Antonín Dvořák's trio for two violins and viola. This sort of trio is known as a terzet or terzetto, and others who have written trios for this combination include Robert Fuchs (three, two in his opus 61 and one in his opus 107) and Sergei Taneyev (his op. 21.)