Gastarbeiter (German for 'guest worker'; pronounced [ˈɡastˌʔaʁbaɪtɐ] (listen); both singular and plural) are foreign or migrant workers, particularly those who had moved to West Germany between 1955 and 1973, seeking work as part of a formal guest worker program (Gastarbeiterprogramm). As a result, guestworkers are generally considered temporary migrants because their residency in the country of immigration is not yet determined to be permanent.^: 87 Other countries had similar programs: in the Netherlands and Belgium it was called the gastarbeider program; in Sweden, Denmark, Norway and Finland it was called arbetskraftsinvandring (workforce-immigration); and in East Germany such workers were called Vertragsarbeiter. The term that was used during the Nazi era was Fremdarbeiter (German for 'foreign worker'). However, the latter term had negative connotations, and was no longer used after World War II.

The term is widely used in Russia (Russian: гастарбайтер, gastarbayter) to refer to foreign workers from post-USSR or third-world countries.