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.

Experimental musical instruments are made from a wide variety of materials, using a range of different sound-production techniques. Some of the simplest instruments are percussion instruments made from scrap metal, like those created by German band Einstürzende Neubauten. Some experimental hydraulophones have been made using sewer pipes and plumbing fittings.

Some experimental musical instruments are created by luthiers, who are trained in the construction of string instruments. Some custom made string instruments are employed with three bridges, instead of the usual two (counting the nut as a bridge). By adding a third bridge, one can create a number of unusual sounds reminiscent of chimes, bells or harps. A 'third bridge instrument' can be a "prepared guitar" modified with an object — for instance, a screwdriver — placed under the strings to act as a makeshift bridge, or it can be a custom made instrument.

Experimental luthiers are luthiers who take part in alternate stringed instrument manufacturing (such as the guitar, violin) or create original string-instruments altogether. Notable experimental luthiers include Yuri Landman, Bradford Reed, and Hans Reichel.

See also: Experimental Musical Instruments