The Spanish Legion , formerly Spanish Foreign Legion, is an elite unit of the Spanish Army

Spanish Army

The Spanish Army is the terrestrial army of the Spanish Armed Forces responsible for land-based military operations. It is one of the oldest active armies - dating back to the 15th century.-Introduction:...

 and Spain's Rapid Reaction Force. Founded as the Tercio

Tercio

The tercio was a Renaissance era military formation made up of a mixed infantry formation of about 3,000 pikemen, swordsmen and arquebusiers or musketeers in a mutually supportive formation. It was also sometimes referred to as the Spanish Square...

 de Extranjeros ("Foreigners Regiment"), it was originally intended as a Spanish equivalent of the French Foreign Legion

French Foreign Legion

The French Foreign Legion is a unique military service wing of the French Army established in 1831. The foreign legion was exclusively created for foreign nationals willing to serve in the French Armed Forces...

, but in practice it recruited almost exclusively Spaniards.
The Spanish Foreign Legion was formed by royal decree of King Alfonso XIII on January 28, 1920, with the Minister of War José Villalba Riquelme stating, "With the designation of Foreigners Regiment there will be created an armed military unit, whose recruits, uniform and regulations by which they should be governed will be set by the minister of war." In the 1920s the Spanish Foreign Legion's five battalions were filled primarily by native Spaniards (since foreigners were not easy to recruit) with most of its foreign members coming from the now independent Republic of Cuba.
Historically there had been a Spanish Foreign Legion which preceded the modern Legion's formation in 1920.