I-21 was a Japanese Type B1 submarine which saw service during World War II in the Imperial Japanese Navy. She displaced 1,950 tons and had a speed of 24 knots (44 km/h). I-21 was the most successful Japanese submarine to operate in Australian waters, participating in the attack on Sydney Harbour in 1942 and sinking 44,000 tons of Allied shipping during her two deployments off the east coast of Australia.

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.