A border blaster is a broadcast station that, though not licensed as an external service, is, in practice, used to target another country. The term "border blaster" is of North American origin, and usually associated with Mexican AM stations whose broadcast areas cover large parts of the United States, and United States border AM stations covering large parts of Canada. Conceptually similar European broadcasting included some pre-World War II broadcasting towards the United Kingdom, "radio périphérique" around France and the U.S. government-funded station Radio Free Europe, targeting European countries behind the Iron Curtain.

With broadcasting signals far more powerful than those of U.S. stations, the Mexican border blasters could be heard over large areas of the U.S. from the 1940s to the 1970s, often to the great irritation of American radio stations, whose signals could be overpowered by their Mexican counterparts. These are also sometimes referred to as X stations for their call letters: Mexico assigns callsigns beginning with XE or XH to broadcast stations.

On November 9, 1972, in Washington, D.C., the United States and Mexico signed an "Agreement Concerning Frequency Modulation Broadcasting in the 87.5 to 108 MHz Band". Since then, in the FM band power levels and frequency assignments have been set by mutual agreement between the two countries. AM radio border blasters still exist, though they are largely ignored due to the decline of AM radio in the U.S. and in Mexico. There are several such stations licensed by Mexico's Secretariat of Communications and Transportation using transmitters with an effective radiated power similar to those of major licensed commercial stations located within the U.S.

In contrast to pirate radio stations which broadcast illegally, border blasters are generally licensed by the government upon whose soil they are located. Pirate radio stations are freebooters from offshore, outside the territorial waters of the nation they target, or ones that are illegally operating in defiance of national law within its sovereign territory. They also contrast with shortwave radio broadcasters, which operate on frequencies expressly designated for international broadcasts, whereas border blasters use frequencies designated for domestic broadcasts.

In Mexico and the US, while the federal government of the US did not particularly like them, the stations were allowed to flourish. W. Lee O'Daniel used a border blaster in his successful campaign for governor of Texas. The US, unlike the UK, has never required a license to listen to broadcast radio or television. The only restriction placed upon border-blasters was a law which prohibited studios in the US from linking by telephone to border-blaster transmitters in Mexico. This law, part of the Brinkley Act, was introduced in the wake of John R. Brinkley's fraudulent medical advice program on XERA. The Brinkley Act remains on the books in the US, but licenses under that act are now routinely granted as long as the station follows applicable US and Mexican regulations.

The pop culture inspired by the border blaster stations is extensive: the 1971 Doors song "The WASP (Texas Radio and the Big Beat)", ZZ Top's song "Heard It on the X" (1975), "The Wolfman of Del Rio" by Terry Allen on his 1979 album Lubbock (On Everything), 1983's "Mexican Radio" by Wall of Voodoo, and 1987's "Border Radio" movie theme by The Blasters.

A similar situation developed in Europe, beginning with Radio Luxembourg after World War II. The British government identified these stations as pirates because the Sunday broadcast was reserved for British listeners (deliberately coinciding with the BBC Sundays of religious programmes). The broadcasts were considered illegal on British soil as these stations were breaking the monopoly of the non-commercial BBC. (Coincidentally, a large percent of the Republic of Ireland could receive spillover from Northern Ireland, Wales and the west of England BBC TV and radio broadcasts for decades.) Listening to the broadcasts was technically a violation of UK radio-license laws of the day. The same "radio périphérique" , or "peripheral radio", phenomenon existed in France from the 1930s until the legalization of private broadcasting in the early 1980s, which allowed Radio Luxembourg from Luxembourg, Radio Andorre and Sud Radio from Andorra, Radio Monte Carlo from Monaco, and Europe 1 from Saarland, Germany, to begin legally broadcasting signals across international borders.

The British government created countermeasures after World War II: the state-owned telephone monopoly prevented studios in Britain from linking by telephone to the transmitters of Radio Luxembourg. These restrictions were mostly lifted following the privatisation and demonopolisation of the UK telephone system.