The I-400-class submarine (伊四百型潜水艦, I-yon-hyaku-gata sensuikan) Imperial Japanese Navy (IJN) submarines were the largest submarines of World War II, with the first one completed just a little over a month before the end of the war. The I-400s remained the largest submarines ever built until the construction of nuclear ballistic missile submarines in the 1960s. The IJN called this type of submarine Sentoku type submarine (潜特型潜水艦, Sen-Toku-gata sensuikan, Submarine Special). The type name was shortened to Toku-gata Sensuikan (特型潜水艦, Special Type Submarine). They were submarine aircraft carriers able to carry three Aichi M6A Seiran aircraft underwater to their destinations. They were designed to surface, launch their planes, then quickly dive again before they were discovered. They also carried torpedoes for close-range combat.

The I-400 class was designed with the range to travel anywhere in the world and return. A fleet of 18 boats was planned in 1942, and work started on the first in January 1943 at the Kure, Hiroshima arsenal. Within a year the plan was scaled back to five, of which only three (I-400 at Kure, and I-401 and I-402 at Sasebo) were completed.