A fretless guitar is a guitar without frets. It operates in the same manner as most other stringed instruments and traditional guitars, but does not have any frets to act as the lower end point (node) of the vibrating string. On a fretless guitar, the vibrating string length runs from the bridge, where the strings are attached, all the way up to the point where the fingertip presses the string down on the fingerboard. Fretless guitars are fairly uncommon in most forms of western music and generally limited to the electrified instruments due to decreased acoustic volume and sustain in fretless instruments. However, the fretless bass guitar has gained fairly widespread popularity and many models of bass guitar can be found in fretless varieties. Fretless Electric Bass is particularly popular among Jazz, Funk and R&B players due to the similarity in feel and sound to the acoustic double bass.

Fretless guitars are not constrained with particular musical tunings, tuning systems or temperaments, as is the case with fretted instruments. This facilitates the playing of music in other than 12-tone scales; these scales are typically found in non-Western or experimental music. Fretless guitars produce a different sound than their fretted counterparts as well, because the fingertip is relatively soft (compared to a fret) and absorbs energy from the vibrating string much faster. The result is that the pizzicato on a fretless guitar has a more damped sound. One can finger notes with their nail like an Indian sarod player does. This will sustain and brighten the sound.

However, playing a fretless instrument usually requires much more training of the fretting hand for exact positioning and shifts, and more ear training to discern the minute differences in intonation that fretless instruments permit. To make this easier, many fretless guitars and basses have lines in place of frets and side position markers (dots or lines), indicating half-tone increments.

Acoustic fretless guitars produce less volume than their fretted counterparts, which is usually addressed by the use of pickups and amplification. Fretless bass guitars, which have much heavier strings and a bigger body, are also typically amplified.

On fretless basses the fingerboard is usually made of a hard wood, such as ebony. To reduce fingerboard wear from round-wound strings a coat of epoxy may be applied. Other strings, such as flat-wound, ground wound or nylon tape-wound strings, can also be used to reduce fingerboard wear.