The third USS Merrimack (AO-37) (ex-Caddo) was one of five Kennebec-class fleet oilers (also known as a type T2 tanker) built during World War II for service in the United States Navy.

Merrimack was laid down as SS Caddo under Maritime Commission contract on 12 September 1940 by Bethlehem Steel Company, Sparrows Point, Maryland.

Merrimack's most memorable crossing began on 23 October 1942 from Hampton Roads, Virginia, when she sailed with the Southern Attack Group of the Western Naval Task Force for Operation Torch, the invasion of North Africa.

For more than a year and a half after the landings in Morocco, Merrimack carried oil to support operations in North Africa, Sicily, Italy, and France.

On her transatlantic voyages, besides oil, she carried passengers and a wide variety of equipment including PT boats, patrol craft, and aircraft.

During the fight for Okinawa, Merrimack alternated between fueling ships involved directly in the landings and the aircraft carriers during raids to on the Japanese Home Islands.

She was stricken from the Navy List on 4 February 1959, transferred to the Maritime Administration (MARAD), and placed in the National Defense Reserve Fleet at Beaumont, Texas.