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 , and after five years with 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.