A talk box (also spelled talkbox and talk-box) is an effects unit that allows musicians to modify the sound of a musical instrument by shaping the frequency content of the sound and to apply speech sounds (in the same way as singing) onto the sounds of the instrument. Typically, a talk box directs sound from the instrument into the musician's mouth by means of a plastic tube adjacent to a vocal microphone. The musician controls the modification of the instrument's sound by changing the shape of the mouth, "vocalizing" the instrument's output into a microphone.

A talk box is usually an effects pedal that sits on the floor and contains a speaker attached with an airtight connection to a plastic tube; however, it can come in other forms, including homemade, usually crude, versions, and higher quality custom-made versions. The speaker is generally in the form of a compression driver, the sound-generating part of a horn loudspeaker with the horn replaced by the tube connection.

The box has connectors for the connection to the speaker output of an instrument amplifier and a connection to a normal instrument speaker. A foot-operated switch on the box directs the sound either to the talk box speaker or to the normal speaker. The switch is usually a push-on/push-off type. The other end of the tube is taped to the side of a microphone, extending enough to direct the reproduced sound in or near the performer's mouth.

When activated, the sound from the amplifier is reproduced by the speaker in the talk box and directed through the tube into the performer's mouth. The shape of the mouth filters the sound, with the modified sound being picked up by the microphone. The shape of the mouth changes the harmonic content of the sound in the same way it affects the harmonic content generated by the vocal folds when speaking.

The performer can vary the shape of the mouth and position of the tongue, changing the sound of the instrument being reproduced by the talk box speaker. The performer can mouth words, with the resulting effect sounding as though the instrument is speaking. This "shaped" sound exits the performer's mouth, and when it enters a microphone, an instrument/voice hybrid is heard.

The sound can be that of any musical instrument, but the effect is most commonly associated with the guitar. The rich harmonics of an electric guitar are shaped by the mouth, producing a sound very similar to voice, effectively allowing the guitar to appear to "speak".

The effect produced by talk boxes and vocoders are often conflated by listeners. However, they have radically different mechanisms for achieving the effect. Talk boxes send the carrier signal into the singer's mouth, where it is then modulated by the singer themselves. On the other hand, vocoders process both the carrier and the modulator signal integrally, producing the output as a separate electric signal. They are also more common in different genres: a talk box is often found in rock music due to its typical pairing with a guitar, whereas vocoders are almost always paired with synthesizers, and as such, are ubiquitous in electronic music.

In 1939, Alvino Rey, amateur radio operator W6UK, used a carbon throat microphone wired in such a way as to modulate his electric steel guitar sound. The mic, originally developed for military pilot communications, was placed on the throat of Rey's wife Luise King (one of The King Sisters), who stood behind a curtain and mouthed the words, along with the guitar lines. The novel-sounding combination was called "Singing Guitar", and employed on stage and in the movie Jam Session, as a "novelty" attraction, but was not developed further.