After discharging her cargo at Sarmi, "Ro-104" began her fifth war patrol on 27 October 1943, proceeding directly from Sarmi to a patrol area in the northern Solomon Islands off Bougainville. After an erroneous report of an Allied landing on Mono Island in the Treasury Islands, she received orders on 31 October 1943 to attack the supposed landing force there. On 1 November 1943, however, the Bougainville campaign began with U.S. landings at Cape Torokina on Bougainville, and a Japanese attempt to bring reinforcements to Bougainville resulted in the Battle of Empress Augusta Bay on 2 November 1943. The Japanese light cruiser was sunk during the battle, and on 3 November 1943 "Ro-104" arrived on the scene and rescued 75 survivors from "Sendai", including the commander of Destroyer Squadron 3, Rear Admiral Matsuji Ijuin. She arrived at Rabaul on 5 November 1943.