Completed before the start of the Pacific War, "Taiyō" ferried aircraft, supplies, and personnel between Japanese bases and trained naval aviators in between transport missions. On 15 July, the ship and her sister "Un'yō" were assigned to the Combined Fleet. Upon receiving news of the American landings on Guadalcanal on 7 August, "Taiyō" and the battleship together with the 2nd and 3rd Fleets sailed from Japan bound for Truk on 17 August. "Taiyō" was detached from the fleet to deliver aircraft to the Marshall Islands on 27 August and then returned to Truk. In September the ship transferred supplies and equipment from Truk to the Philippines, Formosa and Palau; on the return voyage she was torpedoed by an American submarine. After receiving emergency repairs at Truk, the ship was sent to Japan for permanent repairs in October.