The Auxiliary cruiser Mar Negro was an armed merchantman of the Nationalist Spanish navy during the Spanish Civil War. The cargo ship was launched in 1930 along with her sister ship Mar Cantábrico, and after five years on the Compañía Marítima Del Nervión company, she was first requisitioned by the Spanish Republican Navy in 1936. Captured by a group of Nationalist sympathizers from her crew off Algeria in 1937, she entered in service in 1938 after being converted to an auxiliary cruiser.