A MIDI-THRU (output) is an auxiliary output for MIDI signals, mostly found on musical instruments like synthesizers. It started to be added soon after the introduction of MIDI into the market, in order to improve performance.

MIDI consists of a ring of cables, with a transceiver between each cable inside each device. The transceivers physically separate the two cables, meaning that signals not intended for that device have to be re-sent onto the output line. This introduces a delay, one that is long enough to become audible on larger MIDI rings.

The MIDI-THRU port avoids this delay by linking the THRU port to the MIDI-IN socket almost directly. The MIDI-OUT port is then used only for signals originating in that device.

Inexpensive opto-isolators are commonly used in the repeater circuit for MIDI-THRU, so small analog distortion can be expected at the output. Therefore, chaining many devices using MIDI-IN to MIDI-THRU can eventually result in the MIDI signal degrading. This would only occur when the accumulated analog distortion accumlates to the point of becoming digitally significant.