A transmitter is an electronic device that accepts signals or data and translates them into a form that can be transmitted over a medium, usually over a distance. The medium can be wireless or wired, and examples of transmitters include radio transmitters, ultrasonic transducers, line drivers, circuits that drive interfaces, and fiber optic devices that emit light pulses.

A transmitter generates a radio frequency alternating current, which is applied to an antenna to produce radio waves. The purpose of most transmitters is to communicate information over a distance. Transmitters are necessary component parts of all electronic devices that communicate by radio, such as radio and television broadcasting stations, cell phones, walkie-talkies, wireless computer networks, Bluetooth-enabled devices, garage door openers, two-way radios in aircraft, ships, spacecraft, radar sets, and navigational beacons.

A transmitter can be a separate piece of electronic equipment or an electrical circuit within another electronic device. A transmitter and a receiver combined in one unit is called a transceiver. The term transmitter is often abbreviated as "XMTR" or "TX" in technical documents.