The Mellotron is an electro-mechanical, polyphonic keyboard originally developed and built in Birmingham, England in the early 1960s.

The Mellotron superseded the Chamberlin, which was the world's first sample-playback keyboard. The heart of the instrument is a bank of magnetic audio tapes (which are parallel linear, not looped as has sometimes been reported or presumed). Each tape has approximately eight seconds of playing time. Playback heads (underneath each key) enable the playing of pre-recorded sounds.

The earlier MKI and MKII models contained two side-by-side keyboards: On the right keyboard were 18 selectable "lead/instrument" sounds (such as strings, flutes, and brass instruments). The left keyboard played pre-recorded musical rhythm tracks (in various styles).

The tape banks for the later, lighter-weight M400 models contain only 3 selectable sounds such as strings, cello, and the famous eight-voice choir. The sound on each individual tape piece was recorded at the pitch of the key to which it was assigned. To make up for the fewer sounds available, the M400 tapes came in a removable frame, which allowed for relatively quick changes to new racks of sounds.