The viol (), viola da gamba (Italian: [ˈvjɔːla da (ɡ)ˈɡamba]), or informally gamba, is any one of a family of bowed, fretted, and stringed instruments with hollow wooden bodies and pegboxes where the tension on the strings can be increased or decreased to adjust the pitch of each of the strings. Frets on the viol are usually made of gut, tied on the fingerboard around the instrument's neck, to enable the performer to stop the strings more cleanly. Frets improve consistency of intonation and lend the stopped notes a tone that better matches the open strings. Viols first appeared in Spain in the mid-to-late 15th century, and were most popular in the Renaissance and Baroque (1600–1750) periods. Early ancestors include the Arabic rebab and the medieval European vielle, but later, more direct possible ancestors include the Venetian viole and the 15th- and 16th-century Spanish vihuela, a six-course plucked instrument tuned like a lute (and also like a present-day viol) that looked like but was quite distinct from (at that time) the four-course guitar (an earlier chordophone).

Although bass viols superficially resemble cellos, viols are different in numerous respects from instruments of the violin family: the viol family has flat rather than curved backs, sloped rather than rounded shoulders, c holes rather than f holes, and five to seven rather than four strings; some of the many additional differences are tuning strategy (in fourths with a third in the middle—similar to a lute—rather than in fifths), the presence of frets, and underhand ("German") rather than overhand ("French") bow grip.

All members of the viol family are played upright (unlike the violin or the viola, which is held under the chin). All viol instruments are held between the legs like a modern cello, hence the Italian name viola da gamba (it. "viol for the leg") was sometimes applied to the instruments of this family. This distinguishes the viol from the modern violin family, the viola da braccio (it. "viol for the arm"). A player of the viol is commonly known as a gambist, violist , or violist da gamba. "Violist" shares the spelling, but not the pronunciation, of the word commonly used since the mid-20th century to refer to a player of the viola. It can therefore confuse if used in print where context does not indicate that a viol player is meant, though it is entirely unproblematic, and common, in speech.