I-26 was an Imperial Japanese Navy B1 type submarine commissioned in 1941. She saw service in the Pacific War theatre of World War II, patrolling off the West Coast of Canada and the United States, the east coast of Australia, and Fiji and in the Indian Ocean and taking part in Operation K, preparatory operations for the Aleutian Islands campaign, and the Guadalcanal campaign, the Marianas campaign, and the Battle of Leyte Gulf. She was the first Japanese submarine to sink an American merchant ship in the war, sank the first ship lost off the coast of State of Washington during the war, damaged the aircraft carrier USS Saratoga (CV-3), sank the light cruiser USS Juneau (CLAA-52), and was the third-highest-scoring Japanese submarine of World War II in terms of shipping tonnage sunk. Her bombardment of Vancouver Island in 1942 was the first foreign attack on Canadian soil since 1870. In 1944, I-26′s crew committed war crimes in attacking the survivors of a ship she sank. She was sunk in November 1944 during her ninth war patrol.

The Type B1 submarine , also called I-15-class submarine was the first group of boats of the Type B cruiser submarines built for the Imperial Japanese Navy (IJN) during the 1940s. In total 20 were built, starting with I-15, which gave the series their alternative name.