At the end of 1944, the Imperial Japanese Navy decided it needed large numbers of high-speed coastal submarines to defend the Japanese Home Islands^[1] against an anticipated Allied invasion (named Operation Downfall by the Allies). To meet this requirement, the Ha-201-class submarines were designed as small, fast submarines^[1] incorporating many of the same advanced ideas implemented in the German Type XXI and Type XXIII submarines. They were capable of submerged speeds of almost 14 knots (26 km/h; 16 mph).^[1]

The Ha-201 class displaced 325 metric tons (320 long tons) surfaced and 447 metric tons (440 long tons) submerged.^[1] The submarines were 53 meters (173 ft 11 in) long, had a beam of 4.00 meters (13 ft 1 in) and a draft of 3.44 meters (11 ft 3 in).^[1] For surface running, the submarines were powered by a single 400-brake-horsepower (298 kW) diesel engine that drove one propeller shaft.^[1] When submerged the propeller was driven by a 1,250-shaft-horsepower (932 kW) electric motor.^[1] They could reach 11.8 knots (21.9 km/h; 13.6 mph) on the surface and 13.9 knots (25.7 km/h; 16.0 mph) submerged.^[1] On the surface, the Ha-201-class submarines had a range of 3,000 nautical miles (5,600 km; 3,500 mi) at 10 knots (19 km/h; 12 mph); submerged, they had a range of 105 nmi (194 km; 121 mi) at 2 knots (3.7 km/h; 2.3 mph).^[1] Their armament consisted of two 533-millimeter (21 in) torpedo tubes with four torpedoes and a single mount for a 7.7-millimeter machine gun.^[1]

Ha-203 was laid down on 5 April 1945 by the Sasebo Naval Arsenal at Sasebo, Japan, as Submarine No. 4913.^[2] She was renamed Ha-203 on 1 May 1945 and was attached provisionally to the Sasebo Naval District that day.^[2] Launched on 25 May 1945,^[2] she was completed and commissioned on 20 June 1945.^[2]

Upon commissioning, Ha-203 was attached formally to the Sasebo Naval District and assigned to Submarine Division 33 in the Kure Submarine Flotilla for workups.^[2] She had not yet conducted an operational patrol when hostilities between Japan and the Allies ended^[1] on 15 August 1945. She surrendered to the Allies at Sasebo on 2 September 1945.^[2] On 2 November 1945, she was reassigned to Japanese Submarine Division Three under United States Navy command along with her sister ships Ha-201, Ha-202, Ha-205, and Ha-210.^[2]

The Japanese struck Ha-203 from the Navy list on 30 November 1945.^[2] She was among a number of Japanese submarines the U.S. Navy scuttled off the Goto Islands in Operation Road's End on 1 April 1946, sinking just beyond the 100-fathom (600 ft; 183 m) line at 32°37′N 129°17′E / 32.617°N 129.283°E / 32.617; 129.283 (Ha-203).^[2]

• Hackett, Bob; Kingsepp, Sander (2019). "IJN Submarine HA-203: Tabular Record of Movement". SENSUIKAN! Stories and Battle Histories of the IJN's Submarines. Combinedfleet.com. Retrieved 9 October 2020.
• Hackett, Bob; Sander Kingsepp (2015). "Sen Taka Sho Type". Sensuikan!. Combinedfleet.com. Retrieved 9 October 2020.
• "Rekishi Gunzō"., History of Pacific War Vol.17 I-Gō Submarines, Gakken (Japan), January 1998, ISBN 4-05-601767-0
• Rekishi Gunzō, History of Pacific War Extra, "Perfect guide, The submarines of the Imperial Japanese Forces", Gakken (Japan), March 2005, ISBN 4-05-603890-2
• The Maru Special, Japanese Naval Vessels No.43 Japanese Submarines III, Ushio Shobō (Japan), September 1980, Book code 68343-43
• The Maru Special, Japanese Naval Vessels No.132 Japanese Submarines I "Revised edition", Ushio Shobō (Japan), February 1988, Book code 68344-36
• Ships of the World special issue Vol.37, History of Japanese Submarines, "Kaijinsha"., (Japan), August 1993