A hurdy gurdy (alternately, hurdy-gurdy) is a stringed musical instrument. It consists of several strings arranged such that they can all be vibrated by a rotating wheel covered with rosin. This method of producing sound is similar to string instruments such as the violin; but because the hurdy gurdy produces several notes, a melody accompanied by chords made by "drone strings", its sound is perhaps more comparable to that of bagpipes. For this reason, the hurdy gurdy is often accompanied by the bagpipes, particularly in French and Hungarian music.

The earliest form of the hurdy gurdy was the organistrum, a large instrument played by two people, one to turn the crank and the other to pull the keys upward. Because of the great force involved in moving the keys, only slow tunes were playable. The first record of the organistrum is in 12th-century Spain.

Subsequent iterations of the hurdy gurdy were reduced in size to allow one person to play the instrument and switched the key mechanism to require pushing rather than pulling, a much more elegant mechanism that facilitated more complex playing techniques.

Some types of hurdy gurdy, notably the French vielle á roue and the Hungarian tekerõ, also added a buzzing bridge or dog (chien) to one or more drone strings. This mechanism consists of a bridge for the drone string, in this case called the trompette, that is loosely anchored on one side and free on the end the string rests on. Normally the pressure of the string holds the bridge in place, but when the instrument is cranked with greater velocity, the bridge lifts up and buzzes against the soundboard, producing a characteristic rhythmic buzz that is used to provide percussive effect, especially in dance pieces.

The drone strings just produce sounds at fixed pitches, while the other strings are played by tangents, key mechanisms which stop each string, changing the string's length, and hence its pitch. The tangents of the modern hurdy gurdy are arranged according to the Pythagorean system.