A twelve-string guitar (or 12-string guitar) is a steel-string guitar with 12 strings in six courses, which produces a thicker, more ringing tone than a standard six-string guitar. Typically, the strings of the lower four courses are tuned in octaves, with those of the upper two courses tuned in unison. The gap between the strings within each dual-string course is narrow, and the strings of each course are fretted and plucked as a single unit. The neck is wider, to accommodate the extra strings, and is similar to the width of a classical guitar neck. The sound, particularly on acoustic instruments, is fuller and more harmonically resonant than six-string instruments. The 12-string guitar can be played like a 6-string guitar as players still use the same notes, chords and guitar techniques like a standard 6-string guitar, but advanced techniques can be challenging as players need to play or pluck two strings simultaneously.

Structurally, 12-string guitars, especially those built before 1970, differ from six-string guitars in the following ways:

Twelve-string guitars are made in both acoustic and electric forms. However, the acoustic type is more common.

The double ranks of strings of the 12-string guitar produce a shimmering effect, because even the strings tuned in unison can never vibrate with precise simultaneity—that is, they vibrate out of phase. The result to the ear is a sound that seems to "shimmer", which some describe as resembling strings that are slightly detuned. The interference between the out-of-phase vibrations produces a phenomenon known as a beat that results in a periodic rise and fall of intensity which is, in music, often considered pleasing to the ear. Pete Seeger described the distinctive sound of the 12-string guitar as "the clanging of bells."

The origin of the modern 12-string guitar is not clear, but it arose in the closing years of the 19th century. The most likely ancestors using courses of doubled strings are some Mexican instruments of such as the bandolón, the guitarra séptima, the guitarra quinta huapanguera, and the bajo sexto. Pictures such as the 1901 Mexican Typical Orchestra at the Pan-American Exposition show a chordophone with 12 strings.

In the 19th and early 20th centuries, 12-strings were regarded as "novelty" instruments. The 12-string guitar did not become a major part of blues and folk music till the 1920s and the 1930s, when their "larger than life" sound made them ideal as solo accompaniment for vocalists, especially Lead Belly and Blind Willie McTell. The 12-string guitar has since occupied roles in certain varieties of folk, rock, jazz, and popular music. In the 1930s, Mexican-American guitarist and singer Lydia Mendoza popularized the instrument. In the 1950s, Lead Belly's protégé, Fred Gerlach, brought the instrument into the American folk music world. Initially, it was primarily used for accompaniment, owing to the greater difficulty of picking or executing string "bends" on its double-strung courses. The Delta Blues guitar virtuoso Robert Lockwood Jr was presented a handcrafted acoustic 12-string guitar made by an outstanding Japanese luthier in the end of the sixties, and this became the instrument of choice for Lockwood thereafter. In the later 20th century, however, a number of players devoted themselves to producing solo performances on the 12-string guitar, including Roger Hodgson, Leo Kottke, Peter Lang, John McLaughlin, Larry Coryell, Ralph Towner, Robbie Basho, Roger Whittaker, Jack Rose, and James Blackshaw.

Electric 12-strings became a staple in pop and rock music in the 1960s. Early use of the instrument was pioneered by the studio session guitarists of the Wrecking Crew; in 1963, Carol Kaye used a converted Guild six-string on the Crystals' hit "Then He Kissed Me", and on Jackie DeShannon's song "When You Walk in the Room". Glen Campbell played a well-known guitar figure, composed by DeShannon, on an electric 12-string.

One of the first mass-produced electric 12-strings was the Bellzouki. Introduced by Danelectro in 1961 from a design by session guitarist Vinnie Bell, it was initially considered a cross between an electric guitar and a bouzouki rather than an electric version of the traditional 12-string guitar. In the UK in 1963, JMI briefly produced the Vox Bouzouki, later produced in Italy as The Vox Tempest XII, which was used by Vic Flick on the Peter and Gordon hit single "A World Without Love" in 1964. In late 1963, Burns developed the Double Six, supplying a prototype to Hank Marvin of the Shadows, who used it on a number of songs for the soundtrack of the 1964 Cliff Richard movie Wonderful Life; the Double Six was also used on the Searchers' cover version of De Shannon's "When You Walk in the Room."