Completed and commissioned in October 1942, she served in World War II, operating in the Solomon Islands, Rabaul, and New Guinea areas and sinking two cargo ships.

[1] For surface running, the boats were powered by two 500-brake-horsepower (373 kW) diesel engines, each driving one propeller shaft.

[4] In the Battle of the Bismarck Sea, fought between 2 and 4 March 1943, United States Army Air Forces and Royal Australian Air Force aircraft and United States Navy PT boats annihilated a Japanese convoy in the Bismarck Sea that was attempting to carry the Imperial Japanese Army's 51st Division to Lae on New Guinea, sinking all eight ships of the convoy and four of the eight destroyers escorting them.

[4] Ro-103 was in the Solomon Sea off Kiriwina in the Trobriand Islands when she ran aground in darkness on 8 March 1943 on an uncharted reef at 08°20′S 150°45′E / 8.333°S 150.750°E / -8.333; 150.750 (Ro-103).

[4] Meanwhile, Ro-103's crew lightened her by dumping food, supplies, and torpedoes overboard, but she remained aground.

[4] After Ro-103's crew dumped diesel fuel and fresh water overboard to further lighten her, Ro-103 finally floated free of the reef on 11 March 1943.

[4] After the Combined Fleet initiated Operation I-Go — a reinforcement of the 11th Air Fleet base at Rabaul by planes from the aircraft carriers Zuikaku and Zuihō and of the Japanese naval air base on Balalae Island in the Shortland Islands by planes from the aircraft carriers Hiyō and Jun'yō[4] — Ro-103 departed Rabaul on 30 March 1943 in company with the submarine Ro-102 to support the operation by patrolling in the vicinity of Guadalcanal in the southeastern Solomon Islands.

[4] She was 50 nautical miles (93 km; 58 mi) south of the eastern tip of San Cristobal on 23 June 1943 when she sighted a convoy of what she identified as three transports escorted by three destroyers.

[4] On 29 June 1943, Ro-103 was on the surface after sunset recharging her batteries when she sighted seven Allied ships south of Gatukai Island.

[4] On 10 August 1943, the Imperial Japanese Navy declared her to be presumed lost in the Solomon Islands with all 43 men on board.