地下电台，又称海盗电台（英语：Pirate radio），是指未经合法申请，未获得主管机关许可执照而播音的广播电台。地下电台对于一般合法电台的影响甚大，一些合法电台常会因为受到地下电台的电波干扰而导致收听者无法正常收听，不过广播数字化能免除频率区段的影响。在大多数国家，地下电台会被主管机关加以取缔。

另外，部分针对特定对象广播的“秘密电台”（一般为反政府势力、异见人士及分离主义或统战宣传）也被视为地下电台（如韩国的一些“对北广播”电台）。

Pirate radio is illegal or unregulated radio transmission for entertainment or political purposes. While pirate just refers to the illegal nature of the broadcasts, there have also been notable pirate offshore radio transmissions.

Pirate radio generally describes the unlicensed broadcast of FM radio, AM radio, or short wave signals over a wide range. In some cases radio stations are deemed legal where the signal is transmitted, but illegal where the signals are received—especially when the signals cross a national boundary. In other cases, a broadcast may be considered "pirate" due to the nature of its content, its transmission format (especially a failure to transmit a station identification according to regulations), or the transmit power (wattage) of the station, even if the transmission is not technically illegal (such as a web cast or an amateur radio transmission). Pirate radio stations are sometimes called bootleg stations (a term especially associated with two-way radio), clandestine stations (associated with heavily politically motivated operations) or free radio stations.