Portunus was laid down as LST-330 by the Philadelphia Naval Shipyard, 12 November 1942; launched 11 February 1943 as Portunus (AGP-4); and commissioned at Baltimore, Maryland, 12 June 1943.
Underway 4 July in convoy with USS Hilo (AGP-2) and 8 units of MTB Squadron 25, Portunus arrived at Mios Woendi in the Schoutens 9 July. Through December, she underwent overhaul at Brisbane, Australia, and on 29 January 1945 returned to Mios Woendi and resumed repair work.
Invasion of the Philippines
On 20 February she got underway for Leyte Island, whence she proceeded to Ilo Ilo, Panay, to support MTB Ron 33 during the assault there and to establish a patrol base after its success. The assault waves met no apparent opposition, the enemy having set fires and demolition charges and evacuated the city.
Okinawa operations
On 2 April Portunus got underway for Samar and Leyte. On 16 April she joined company with the remainder of TG 78.2 to land the 24th Infantry Division, United States Army and secure Parang, Mindanao. She steamed between various points on Mindanao and Samar, supplying MTB's, until 16 July when she got underway in convoy for Okinawa. She anchored off Hagushi, 21 July and shifted to Togouchi Harbor the next day. She serviced and repaired MTB Ron 31 and 37 and various other units through 29 September when she prepared to get underway for California and inactivation.