USS Ajax (AR-6), in service 1943 to 1986, was the second Vulcan-class repair ship and the fourth ship in the United States Navy to bear the name. Laid down in 1941, launched in 1942 and commissioned in 1943, she was decommissioned in 1986 and finally sold for scrap. Ajax received four battle stars for Korean War service and five campaign stars for service in Vietnam.

Ajax was laid down on 7 May 1941 at Los Angeles Shipbuilding and Dry Dock Company, San Pedro, California, launched on 22 August 1942. She was sponsored by Mrs. Isaac C. Johnson and commissioned on 30 October 1943.

The repair ship departed San Pedro on 9 December 1943, arrived at Pearl Harbor on 16 December, and began preparing small craft to be used as control vessels in the Marshall Islands campaign by installing radar, sound detection equipment, and antiaircraft guns. On 8 January 1944, an oil fire in her blacksmith shop threatened the entire ship, but was extinguished. Nevertheless, Ajax spent part of January repairing her own damage.

On 25 January, Ajax was ordered to proceed in company with USS Wadleigh (DD-689) to the Ellice Islands; but, two days after reaching Funafuti, she moved to Makin Atoll, Gilbert Islands, to work on the ships that would occupy Majuro in the Marshall Islands. Upon completing that mission, the ship returned to Funafuti on 26 February, only to sail three days later for Majuro.

While she was serving there, Service Squadron (ServRon) 4 was absorbed by ServRon 10. There, she and Vestal (AR-4) repaired combatant ships through the Hollandia strikes and during preparations for the Marianas campaign. On 13 June, she sailed for Eniwetok to help set up an advance repair base where she labored through August, at one time working extensive jobs on 19 cruisers and nine battleships.

Late in August, bacillary dysentery broke out among the crew and soon reached epidemic proportions. The ship was quarantined on 1 September and detached on 9 September to proceed to Kwajalein to combat the epidemic. Quarantine ended on 10 October, and Ajax steamed to Ulithi to resume repair work and to handle her first major battle damage job. Severely damaged during a torpedo attack off Formosa, Canberra (CA-70) received sufficient temporary repairs alongside Ajax to enable the cruiser to continue on to Manus. The repair ship continued her work at Ulithi in support of operations in the Philippines, Iwo Jima, and Okinawa.

On 25 May 1945, Ajax headed for Leyte-Samar Naval Base in San Pedro Bay, Leyte Gulf, to help prepare for the final assault on Japan, spending July repairing typhoon-battered USS Bennington (CV-20). The job consisted of rebuilding the forward section of her flight deck and required assistance from USS Basilan (AG-68), Baham (AK-122), and USS Jason (ARH-1).

Upon learning of Japan's capitulation on 15 August, Ajax began readying amphibious and transport ships to carry occupation forces to the Japanese home islands. On 20 September, she sailed for Guiuan, Samar, where she embarked troops for passage to Okinawa; once there, she repaired other typhoon-damaged ships. Ironically, while she was carrying out this task, typhoons forced her to go to sea herself on 28 September and on 7 October. But for these two incidents, her work at Okinawa was uninterrupted until 28 November, when she sailed for the United States with 800 passengers. She arrived at San Diego on 18 December and, three days later, entered the San Francisco Naval Shipyard for a six-week overhaul.