The viola ( vee-OH-lə, also UK: vy-OH-lə, Italian: [ˈvjɔːla, viˈɔːla]) is a string instrument that is bowed, plucked, or played with varying techniques. Slightly larger than a violin, it has a lower and deeper sound. Since the 18th century, it has been the middle or alto voice of the violin family, between the violin (which is tuned a perfect fifth above) and the cello (which is tuned an octave below). The strings from low to high are typically tuned to C₃, G₃, D₄, and A₄.

In the past, the viola varied in size and style, as did its names. The word viola originates from the Italian language. The Italians often used the term viola da braccio meaning literally: 'of the arm'. "Brazzo" was another Italian word for the viola, which the Germans adopted as Bratsche. The French had their own names: cinquiesme was a small viola, haute contre was a large viola, and taile was a tenor. Today, the French use the term alto, a reference to its range.

The viola was popular in the heyday of five-part harmony, up until the eighteenth century, taking three lines of the harmony and occasionally playing the melody line. Music for the viola differs from most other instruments in that it primarily uses the alto clef. When viola music has substantial sections in a higher register, it switches to the treble clef to make it easier to read.

The viola often plays the "inner voices" in string quartets and symphonic writing, and it is more likely than the first violin to play accompaniment parts. The viola occasionally plays a major, soloistic role in orchestral music. Examples include the symphonic poem Don Quixote, by Richard Strauss, and the symphony/concerto Harold en Italie, by Hector Berlioz. In the earlier part of the 20th century, more composers began to write for the viola, encouraged by the emergence of specialized soloists such as Lionel Tertis and William Primrose. English composers Arthur Bliss, York Bowen, Benjamin Dale, Frank Bridge, Benjamin Britten, Rebecca Clarke and Ralph Vaughan Williams all wrote substantial chamber and concert works. Many of these pieces were commissioned by, or written for, Lionel Tertis. William Walton, Bohuslav Martinů, Tōru Takemitsu, Tibor Serly, Alfred Schnittke, and Béla Bartók have written well-known viola concertos. The concerti by Béla Bartók, Paul Hindemith, Carl Stamitz, Georg Philipp Telemann, and William Walton are considered major works of the viola repertoire. Paul Hindemith, who was a violist, wrote a substantial amount of music for viola, including the concerto Der Schwanendreher.