The Mellotron was an electromechanical polyphonic keyboard instrument which can be likened to an early synthesiser, in that it re-created the sound of other instruments. But whereas a true synthesiser produces synthetic sounds by electronic means, the Mellotron played recordings of actual instrument sounds.

The Mellotron was originally developed and made by the Bradley family in Birmingham, who set up a company called Mellotronics for this purpose. Early Mellotrons used short loops of tape, each carrying a different sound. For example, one tape loop might have 5 violins playing a note E, with another having 5 violins playing the note G, and so on. When a particular key wa pressed, the tape's playback head was brought into contact with the tape, thus producing that tape's sound.

The early electro-mechanical style of Mellotron described above is now replaced by the all-electronic Mellotron, where the musical instruments are digitally recorded for playback.