An effects pedal (or a "Stomp Box") is an electronic effects unit housed in a small metal or plastic chassis used by musicians, usually electric guitar players, to modify their guitar sound. Musicians playing electronic keyboards, electromechanical organs, the electric bass, or electric violin also use effects pedals. These devices alter the sound quality or timbre of the input signal, adding effects such as distortion, fuzz, overdrive, chorus, reverberation, wah-wah, flanging, phaser or pitch shifting. The sound of a guitar or other instrument that is played without an effects pedal is described as "clean", "straight" or "dry."

They are called pedals because they sit on the floor and have large on/off switches on top that are activated using the foot. Some pedals, such as wah-wah or volume pedals, employ what is known as an expression pedal, which is manipulated while in operation by rocking a large foot-activated (treadle) potentiometer back and forth. The relative position of the expression pedal thus determines the extent to which the sound is altered.

Effects pedals permit the guitarist/bassist to activate and deactivate effects while playing an instrument. Larger rack-mounted effect units are generally more expensive and delicate, but they have superior sound quality, and are used in studio applications for recording. Pedals are intended mainly for use in live performances.