The Leslie speaker is a specially constructed loudspeaker used to create special audio effects. Named after its inventor, Don Leslie, it is particularly associated with the Hammond organ. Separate Leslie speakers were a "must have" accessory for all Hammond owners, particularly after its characteristic sound was popularised by such acts as Procul Harum on A Whiter Shade of Pale or the Spencer Davis Group on Gimme Some Loving.

The Leslie speaker consists of two driver units - a treble unit with horns, and a bass unit. The key feature is that the horns of the treble unit, (the treble unit has only one horn in fact but it looks like two because a dummy horn is used to counter-balance the horn that works) and a sound baffle for the bass unit, are rapidly rotated using electric motors to create vibrato, tremolo and chorus effects. It can be switched between its two speeds, fast/slow and its the transition between the two that produces the most characteristic effects. The resulting sound is instantly identifiable as that of the Hammond organ, frequently heard on psychedelic and rock music of the 1960s and 1970s. Unlike a high fidelity loudspeaker, the Leslie is specifically designed to alter or modify sound, not reproduce it, and so faithful reproduction has never been part of its appeal.

While normally used with the Hammond, because it is a separate unit, any musical source, such as an electric guitar, can be played back through it, creating a wide range of surprising and dramatic effects. The classic Leslie is still made and sold to this day, though in modern times similar effects can be obtained digitally. However, the digital emulators fall short of achieving the Leslie magic completely.