The Infinite Guitar is a modified electric guitar developed by Canadian musician and producer Michael Brook in the early 1980s, designed to allow a note or chord to be held with unlimited sustain. The instrument uses an onboard electronic circuit that takes the signal from a standard magnetic pickup, amplifies it, and feeds it back into a second pickup coil, producing controlled self-oscillation at the string. Brook, who had studied drones and long tones while working with LaMonte Young and had been inspired by seeing Bill Nelson perform with an EBow, built his first prototype on a Tokai Stratocaster copy as a way to get similar sustained textures without external devices.

Only three Infinite Guitars were ever built — one for Brook himself, one for producer Daniel Lanois, and one for The Edge of U2. The Edge's unit became the best-known example after it was used to record the sustained, bell-like high notes on "With or Without You" from U2's 1987 album The Joshua Tree, a part that helped salvage the song during a troubled recording process. Brook also used the instrument extensively on his own ambient and soundtrack work, including collaborations with Brian Eno and Lanois.

Because the Infinite Guitar was never put into commercial production, it exists more as a piece of studio folklore than a catalog product, and the underlying concept went on to influence mass-market sustainer systems such as the Fernandes Sustainer and the Moog Guitar. Questions of priority have also been raised, with earlier sustain-modified instruments (notably work by Randy Roos and Steve Holland in the late 1970s) predating Brook's design.

Infinite Guitar has 9 products cataloged on Equipboard, including Solid Body Electric Guitars and Electric Basses.