A mandolin (Italian: mandolino pronounced [mandoˈliːno]; literally "small mandola") is a stringed musical instrument in the lute family and is generally plucked with a pick. It most commonly has four courses of doubled strings tuned in unison, thus giving a total of eight strings. A variety of string types are used, with steel strings being the most common and usually the least expensive. The courses are typically tuned in an interval of perfect fifths, with the same tuning as a violin (G3, D4, A4, E5). Also, like the violin, it is the soprano member of a family that includes the mandola, octave mandolin, mandocello and mandobass.

There are many styles of mandolin, but the three most common types are the Neapolitan or round-backed mandolin, the archtop mandolin and the flat-backed mandolin. The round-backed version has a deep bottom, constructed of strips of wood, glued together into a bowl. The archtop, also known as the carved-top mandolin has an arched top and a shallower, arched back both carved out of wood. The flat-backed mandolin uses thin sheets of wood for the body, braced on the inside for strength in a similar manner to a guitar. Each style of instrument has its own sound quality and is associated with particular forms of music. Neapolitan mandolins feature prominently in European classical music and traditional music. Archtop instruments are common in American folk music and bluegrass music. Flat-backed instruments are commonly used in Irish, British, and Brazilian folk music, and Mexican estudiantinas.

Other mandolin variations differ primarily in the number of strings and include four-string models (tuned in fifths) such as the Brescian and Cremonese; six-string types (tuned in fourths) such as the Milanese, Lombard, and Sicilian; 6 course instruments of 12 strings (two strings per course) such as the Genoese; and the tricordia, with 4 triple-string courses (12 strings total).

Much of mandolin development revolved around the soundboard (the top). Early instruments were quiet, strung with gut strings, and plucked with the fingers or with a quill. However, modern instruments are louder, using metal strings, which exert more pressure than the gut strings. The modern soundboard is designed to withstand the pressure of metal strings that would break earlier instruments. The soundboard comes in many shapes—but generally round or teardrop-shaped, sometimes with scrolls or other projections. There are usually one or more sound holes in the soundboard, either round, oval, or shaped like a calligraphic f (f-hole). A round or oval sound hole may be covered or bordered with decorative rosettes or purfling.