An FM radio transmitter is a portable device that plugs into the headphone jack or proprietary output port of a portable audio or video device, such as a portable media player, CD player, or satellite radio system. The sound is then broadcast through the FM transmitter, and plays through an FM broadcast band frequency. Purposes for an FM radio transmitter include playing music from a device through a car stereo, or any radio.

The FM-transmitter plugs into the audio output of audio devices and converts the audio output into an FM radio signal, which can then be picked up by appliances such as car or portable radios. Most devices on the market typically have a short range of up to 30 feet (9 meters) (up to about 75 feet (23 meters) with a very good radio under perfect conditions) with any average radio and can broadcast on any FM frequency from 76.0 to 108.0 MHz (or 88.1 to 107.9 in the US). Some lower-cost transmitters are hard-wired to the 87.7-91.9 MHz band allocated to educational broadcasts in the United States, or a certain other smaller range of frequencies.