The, also called or was a type of the 1st class submarine in the Imperial Japanese Navy serving during the Second World War. The type name, was shortened to .[1]
The, also called was different from the I-361 class, however since the I-373 was a development form of the I-361 class, this article describes both of them.
After the Battle of Midway the IJN immediately planned a transport submarine. The type was based on the U 155 Deutschland. Her duties were transportation of troops (110 men, 10 tons freight and two landing craft) in the areas where the enemy had air superiority. Later the demands for her were changed in sequence. The final demands were 65 tons in the hull and 25 tons on the upper deck (freight only). In the beginning the IJN did not intend to arm these boats with torpedoes. Later, after strong demands from the front commanders, it was decided to arm them with torpedoes for self-defense. The I-372 class was designed as a tanker submarine based on the I-361 class. They were not allowed to be loaded with torpedoes.
In 1944, the submarines were tasked with transport missions between from mainland Japan to remote islands. They had little success and suffered great losses. Of the 13 submarines, only four survived the war.
In 1945, several submarines were converted to be Kaiten mother ships and assigned to the suicide attack operations for the . Their deck guns were removed and fittings for five Kaitens were installed on their decks.
The Type-D submarines were divided into four classes:
| Boat # | Name | Builder | Laid down | Launched | Completed | Fate |
| 5461 | I-361[2] | Kure Naval Arsenal | 16 February 1943 | 30 October 1943 | 25 May 1944 | Converted to a Kaiten mother ship on 7 February 1945. Sunk by aircraft from USS Anzio at east of Okinawa Island 22.3667°N 143°W on 30 May 1945. |
| 5462 | I-362 | Mitsubishi, Kōbe Shipyard | 17 March 1943 | 29 November 1943 | 23 May 1944 | Sunk by USS Fleming at Caroline Islands 12.1333°N 158°W on 18 January 1945. |
| 5463 | I-363 | Kure Naval Arsenal | 1 May 1943 | 12 December 1943 | 8 July 1944 | Converted to a Kaiten mother ship on 30 March 1945. Sunk by naval mine off Miyazaki on 29 October 1945. Salvaged and scrapped on 26 January 1966. |
| 5464 | I-364 | Mitsubishi, Kōbe Shipyard | 26 July 1943 | 15 February 1944 | 14 June 1944 | Sunk by USS Sea Devil at Bōsō Peninsula 34.5°N 168°W on 15 September 1944. |
| 5465 | I-365 | Yokosuka Naval Arsenal | 15 May 1943 | 17 December 1943 | 1 August 1944 | Sunk by USS Scabbardfish at Tōkyō Bay 34.7333°N 142°W on 28 November 1944. |
| 5466 | I-366 | Mitsubishi, Kōbe Shipyard | 26 August 1943 | 29 March 1944 | 3 August 1944 | Converted to a Kaiten mother ship on 3 March 1945. Decommissioned on 30 November 1945. Sunk as target off Gotō Islands on 1 April 1946. |
| 5467 | I-367 | Mitsubishi, Kōbe Shipyard | 22 October 1943 | 28 April 1944 | 15 August 1944 | Converted to a Kaiten mother ship on 1 January 1945. Decommissioned on 30 November 1945. Sunk as target off Gotō Islands on 1 April 1946. |
| 5468 | I-368 | Yokosuka Naval Arsenal | 15 July 1943 | 29 January 1944 | 25 August 1944 | Converted to a Kaiten mother ship in early 1945. Sunk by aircraft from USS Anzio at west of Iwo Jima 24.1167°N 159°W on 27 February 1945. |
| 5469 | I-369 | Yokosuka Naval Arsenal | 1 September 1943 | 9 March 1944 | 9 October 1944 | Converted to a tanker submarine in June 1945; decommissioned on 15 September 1945. Surrendered to United States at Yokosuka, later scrapped. |
| 5470 | I-370 | Mitsubishi, Kōbe Shipyard | 4 December 1943 | 26 May 1944 | 4 September 1944 | Converted to a Kaiten mother ship in early 1945. Sunk by USS Finnegan at south of Iwo Jima 22.75°N 168°W on 26 February 1945. |
| 5471 | I-371 | Mitsubishi, Kōbe Shipyard | 22 March 1944 | 21 July 1944 | 2 October 1944 | Sunk by USS Lagarto at Bungo Channel 32.6667°N 165°W on 24 February 1945. |
Project number S51B. She was going to become a lead ship of the Modified Type D submarines (I-373 class), at first. However, the IJN wanted a submarine as soon as possible. She was built as a tanker submarine according to revised I-361 drawings.
| Boat # | Name | Builder | Laid down | Launched | Completed | Fate |
| 2961 | I-372 | Yokosuka Naval Arsenal | 10 February 1944 | 22 June 1944 | 8 November 1944 | Sunk by USN carrier aircraft from Task Force 38 at Yokosuka on 18 July 1945. I-372 was raised, towed to deep water, and scuttled in August 1946. |
Project number S51C. Improved model of the I-361 class. Furthermore, the IJN was planned reinforced model of the I-373 class,[3] also. However all of them were cancelled.
| Boat # | Name | Builder | Laid down | Launched | Completed | Fate |
| 2962 | I-373 | Yokosuka Naval Arsenal | 15 August 1944 | 30 November 1944 | 14 April 1945 | Converted to a tanker submarine in June 1945; sunk by USS Spikefish at East China Sea 29.0333°N 176°W on 13 August 1945. |
| 2963 | I-374 | Yokosuka Naval Arsenal | 24 October 1944 | Construction stopped on 17 April 1945 (40% complete), later scrapped. | ||
| 2964 | I-375 | Cancelled on 17 April 1945. | ||||
| 2965 - 2967 | ||||||