The twelve string guitar is an acoustic or electric guitar with twelve strings, which produces a richer, more ringing tone than a standard six string guitar.

The 12-string guitar is almost essentially in the brazilian style "Viola", played by a majority of rural musicians and the precursor of modern brazilian "sertanejo" music.

Some players, either in search of distinctive tone or for ease of playing, will remove some of the doubled strings. For example, removing the higher octave from the three bass courses simplifies playing running bass lines, but keeps the extra treble strings for the full strums.

The tension placed on the instrument by the strings is great, and because of this, 12 string guitars have a reputation for warping after a few years of use. Some twelve-string guitars have nontraditional structural supports to prevent or postpone such a fate, at the expense of appearance and tone. Until recently, twelve-string guitars were nearly universally tuned lower than the traditional EADGBE, to reduce the stresses on the instrument. Leadbelly famously used a low C-tuning, as did Beau on his Dandelion recordings.

Many performers who play the twelve-string guitar use an ordinary six-string guitar as their primary instrument, switching to the twelve-string guitar for certain songs that seem to call for a brighter sound.

Because it is substantially more difficult to pluck individual strings on the twelve-string guitar, and almost impossible to bend notes tunefully, the instrument is rarely used for lead musical parts. It is primarily suited to a rhythm, accompaniment, or solo role, and is often used in folk songs and some popular music. Some hard rock and progressive rock musicians use double-necked guitars, which have both six-string and twelve-string components, allowing the guitarist easy transition between different sounds.

The greater number of strings complicates playing, particularly for the right hand. The gap between the dual-string courses is usually narrower than that between the single-string courses of a conventional six-string guitar, so more precision is required with pick or fingertip when not simply strumming chords. The pairing of thin, easily broken octave strings with larger, stiffer bass strings presents difficulties to the player also, and only a very skilled player can reliably pluck single strings from within a course at any speed (notably the very high octave G string, which is the highest-pitched string on the instrument). Nevertheless, with practice, the twelve-string guitar is not unduly difficult to play. It is, however, generally used in a fairly restricted role which emphasises its strengths: rich ringing, full-bodied chords, and fast, rippling single plucked notes on the twinned strings.

Twelve-string guitars are made in both acoustic and electric form. However, it is the acoustic type that is most common. Use of twelve-string electric guitar is cyclical: massively popular in the mid-1960s thanks largely to its use by George Harrison of The Beatles and Roger McGuinn of The Byrds, it fell out of favor by the decade's end and was largely limited to niche use by progressive rockers in the 1970s. The instrument was revived in the 1980s by alternative rockersPeter Buck of REM and Johnny Marr of The Smiths. During the 1990s, its popularity waned again, although it plays a key part in the sound of indie rock acts such as Low and The Decemberists. The most popular electric twelve-string model since the 1960s has been the Rickenbacker 360/12, first popularized by Harrison. Many double-neck guitars have a twelve-string neck, in order for guitarists to switch between tones during live performances, for example, when playing 'Stairway to Heaven.'