I-123, originally named Submarine No. 50 and then named I-23 from before her construction began until June 1938, was an I-121-class submarine 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 the Philippines, the Battle of Midway, the Guadalcanal campaign, and the Battle of the Eastern Solomons. She was sunk in 1942. After she was renumbered I-123 in 1938, the number I-23 was assigned to a later submarine which also served during World War II.