1927–1937 Upon commissioning,
I-21 was assigned to either the
Kure Naval District Submarine Division 13 was assigned to the Kure Defense
Squadron in the Kure Naval District on 15 November 1935, On 1 December 1937, Submarine Division 13 was assigned to Submarine Squadron 3 in the
4th Fleet, a component of the
Combined Fleet, On 20 June 1938,
I-121 was placed in the Second Reserve in the Kure Naval District. The four submarines departed
Sasebo, Japan, on 16 May 1940 and visited the waters of the
Caroline Islands,
Marshall Islands, and
Mariana Islands before concluding their cruise with their arrival at
Yokosuka, Japan, on 22 September 1940. Submarine Division 13 was reassigned directly to the Kure Naval District on 15 November 1940, When
I-123 arrived off the French Frigate Shoals in late May 1942, however, she found the U.S. Navy
seaplane tenders and already operating flying boats there, and Operation K-2 was cancelled.
I-121 received
I-123′s report of the U.S. activity at French Frigate Shoals on 30 May 1942. She was informed of the cancellation of Operation K-2 on 31 May 1942, and that day was ordered to continue to patrol in the vicinity of the French Frigate Shoals. The
Battle of Midway began on 4 June 1942, and that day
I-121 and
I-123 were ordered to move westward to new patrol areas. While southwest of
Lisianski Island en route to her new patrol area on 5 June 1942,
I-121 sighted the submarine heading northeast on the surface. At 14:59
local time Dolphin sighted
I-121′s periscope dead astern of her and moved out of range before
I-121 could achieve a firing solution against her. The Battle of Midway ended on 7 June 1942 in a decisive Japanese defeat, and the Japanese cancelled the invasion of Midway.
I-121 concluded her patrol with her arrival at Kwajalein in company with
I-122 and
I-123 on 25 June 1942. She later returned to Japan, arriving at Yokosuka.
Guadalcanal campaign Toward the end of
I-121′s stay at Yokosuka, Submarine Division 13 was resubordinated to Submarine Squadron 7 in the
8th Fleet on 14 July 1942. On 16 July 1942, she departed Yokosuka to take up her new duties, calling at
Truk from 24 to 31 July 1942 and arriving at
Rabaul on
New Britain on 4 August 1942. While
I-121 was at Rabaul, the
Guadalcanal campaign began on 7 August 1942 with U.S. amphibious landings on
Guadalcanal,
Tulagi,
Florida Island,
Gavutu, and
Tanambogo in the southeastern
Solomon Islands.
I-121 and
I-122 put to sea that day from Rabaul to conduct a reconnaissance in the waters off Guadalcanal and Tulagi. The U.S. submarine sighted
I-121 on the surface south of
Cape St. George on
New Ireland on 8 August 1942 but was unable to attack her. From 15 to 17 August 1942,
I-121 reconnoitered
Lungga Roads off Guadalcanal. On 18 August 1942 she moved to a new patrol area southeast of
San Cristobal. and her squadron, Submarine Squadron 13, was resubordinated to the Advance Force on 21 August 1942. In her new patrol area,
I-121 unsuccessfully attacked U.S. Navy
Task Force 16 southeast of San Cristobal on 22 August 1941. One of her torpedoes
broached briefly as it passed between the aircraft carrier and the
heavy cruiser . On 24–25 August 1942, Task Force 16 fought in the
Battle of the Eastern Solomons, in which
Enterprise suffered damage.
I-121 received orders on 26 August 1942 to move south to intercept
Enterprise as
Enterprise withdrew for repairs.
I-121 was on the surface recharging her batteries on 27 August 1942 when two
SBD Dauntless dive bombers from the aircraft carrier attacked her at 06:30. As she submerged, one bomb struck her empty mine storage compartment. The damage caused a serious leak, and a few hours later she surfaced to make emergency repairs. Unable to submerge for the remainder of her patrol despite the repairs, she headed for Rabaul. On 28 August 1942, while northeast of San Cristobal en route Rabaul, she sighted an aircraft carrier and several destroyers at 04:30, and at 08:00 she sighted an aircraft carrier, two
cruisers, and four destroyers, but she arrived at Rabaul without further incident at 15:00 on 4 September 1942. After further repairs,
I-121 got underway from Rabaul on 8 September 1942 bound for Japan. After arriving at Kure on 20 September 1942, she underwent an overhaul.
New Guinea campaign With her repairs and overhaul complete,
I-121 departed Kure on 1 December 1942, stopped at Truk from 10 to 17 December 1942, and arrived at Rabaul on 21 December 1942, where she was reassigned to Submarine Group B. In support of Japanese forces fighting in the
New Guinea campaign, she departed Rabaul on 23 December 1942 to carry supplies to
Buna on the coast of
New Guinea. Reassigned to Submarine Group D, she again got underway from Rabaul on 4 January 1943, patrolling southeast of New Guinea and returning to Rabaul on 25 January 1943. On 29 January 1943, she again put to sea from Rabaul, this time to refuel reconnaissance
floatplanes at the
Indispensable Reefs. She returned from this operation on 10 February 1943. Departing Rabaul on 14 February 1943, she called at Truk from 18 to 23 February, then proceeded to Japan, where she arrived at Kure on 5 March 1943 for an overhaul. While she was at Kure, her squadron, Submarine Squadron 7, was reassigned to the
Southeast Area Fleet. On 25 April 1943,
I-121 departed Kure bound for Rabaul, which she reached on 7 May 1943. She then made her first supply run to
Lae, New Guinea, departing Rabaul on 10 May, arriving at Lae on 14 May, dropping off 26 tons of food and ammunition, embarking 15 soldiers, and departing the same day to return to Rabaul, where she arrived on 17 May 1943. She set out for Lae again on 19 May 1943 for her second supply run, but engine trouble forced her to return to Rabaul on 22 May 1943. She had greater success on her third run, getting underway from Rabaul on 23 May, dropping off 26 tons of food and ammunition at Lae on 26 May, and returning to Rabaul on 29 May 1943. On 31 May 1943, Submarine Division 13 was disbanded, and
I-121 and
I-122 were attached directly to Submarine Squadron 7 headquarters.
I-121 continued making supply runs to Lae throughout June and July and into early August 1943. She started her fourth supply run from Rabaul on 1 June 1943, unloading 26.5 tons of food and ammunition at Lae on 3 June and embarking 15 soldiers, departing the same day, and arriving at Rabaul on 6 June 1943. Her fifth run began on 8 June; she reached Lae on 10 June 1943, again dropped off 26.5 tons of food and ammunition and took aboard 15 soldiers, and left the same day, arriving at Rabaul on 13 June 1943. On her sixth run, she got underway from Rabaul on 20 June 1943 and unloaded 26.5 tons of food and ammunition at Lae on 22 June 1943, leaving for Rabaul the same day. After that, she made three more supply runs in which she called at Lae on 7 July, 27 July, and 3 August 1943. On 15 August 1943,
I-121 was transferred to the Kure Guard Unit in Japan. Before departing for Japan, she made a final supply run to Lae, her tenth, departing Rabaul on 19 August 1943 and arriving at Lae on 20 August. After that she headed for Japan, arriving at Kure on 1 September 1943.
Home waters After arriving in Japan, the aging
I-121 and
I-122, by then considered obsolescent, were withdrawn from combat and assigned duty as
training ships at
Kure Naval Base. By December 1943, they were part of Submarine Division 18 along with the submarines , , and , which also had been relegated to a training role. On 5 January 1944,
I-121 and the submarine took part in the first stage of submarine camouflage pattern experiments conducted by the Naval Submarine School in the
Seto Inland Sea, apparently having their
hull and
conning tower sides painted in a greenish-gray pattern. Between 23 and 25 February 1945, the two submarines took part in the second stage of the experiments, again in the Seto Inland Sea, with the same camouflage pattern extended to their
decks. On 20 April 1945,
I-121 and
I-122 were resubordinated to Submarine Division 33 in the Kure Submarine Squadron.
I-121 was transferred to the
Maizuru Naval Base on 12 June 1945 and remained there through the end of World War II. Hostilities ceased on 15 August 1945, and
Japan formally surrendered on 2 September 1945.
I-121 surrendered to the Allies in September 1945 at
Maizuru. ==Final disposition==