Wetback is a derogatory term for a Mexican laborer who, in order to obtain work in the United States, illegally enters the country by swimming or wading the Rio Grande. The Spanish-language translation espalda mojada – or, more tersely, mojado – is used in a similar sense by Hispanic Americans. Espalda mojada is also used in an equally pejorative way in Spain to refer to illegal immigrants from Morocco who cross the Straits of Gibraltar.

In both the United States and Spain, such economic migrants or foreign workers, in spite of their illegal status, are a major source of cheap unskilled labor.

The English term was first used in 1944, and it saw official use by the U.S. Government in 1954 with Operation Wetback.

See: Marielito, List of ethnic slurs

A wetback stove or wetback heater is the name (used in New Zealand at least) for a simple household secondary water-heater using incidental heat. It typically consists of a hot water pipe running behind a fireplace or stove (rather than hot water storage ), and has no facility to limit the heating. Modern wetbacks may run the pipe in a more sophisticated design to assist heat-exchange.