I-121, laid down in 1924 as Submarine No. 48 and known as I-21 from November 1924 to June 1938, was an of the Imperial Japanese Navy that served during the Second Sino-Japanese War and World War II. During the latter conflict, she conducted operations in support of the Japanese invasion of Malaya, the Japanese invasion of the Philippines, the bombing of Darwin, the Battle of Midway, the Guadalcanal campaign, the Battle of the Eastern Solomons, and the New Guinea campaign. She surrendered at the end of the war in 1945 and was scuttled in 1946.