SS Mission San Diego was a Type T2-SE-A2 tanker built for the United States Maritime Commission during World War II.

Chartered to Deconhill Shipping Co. upon delivery for operations, she spent the remainder of the war carrying fuel to allied forces overseas in the Pacific (during which time she was awarded the National Defense Service Medal).

After 1 October 1949 she was transferred to the newly created Military Sea Transportation Service (MSTS) for duty as USNS Mission San Diego (T‑AO‑121).

Reacquired by the Navy on 3 July 1956 she was placed in service with MSTS, but served only until 16 October 1957 when she was returned to MARAD and laid up in the Maritime Reserve Fleet at James River, Virginia.

Upon completion of the conversion and delivery in 1967 Seatrain Washington, IMO 6704488, was chartered to the MSTS in support of overseas U.S. military operations, including the transport of material, equipment and aircraft to Vietnam.