An experimental musical instrument (or custom-made instrument) is a musical instrument that modifies or extends an existing instrument or class of instruments, or defines or creates a new class of instrument. Some are created through simple modifications, such as cracked drum cymbals or metal objects inserted between piano strings in a prepared piano. Some experimental instruments are created from household items like a homemade mute for brass instruments such as bathtub plugs. Other experimental instruments are created from electronic spare parts, or by mixing acoustic instruments with electric components.

The instruments created by the earliest 20th-century builders of experimental musical instruments, such as Luigi Russolo (1885–1947), Harry Partch (1901–1974), and John Cage (1912–1992), were not well received by the public at the time of their invention. Even mid-20th century builders such as Ivor Darreg, Pierre Schaeffer and Pierre Henry did not gain a great deal of popularity. However, by the 1980s and 1990s, experimental musical instruments gained a wider audience when they were used by bands such as Einstürzende Neubauten and Neptune.