横須賀鎮守府
The submarine was laid down on 7 January 1939 at the Kawasaki shipyard, Kobe, and launched on 24 February 1940. On 15 July 1941 she was completed, commissioned and assigned to Submarine Squadron 1's Submarine Division 3 in the Sixth Fleet. "I-21" was based in the Yokosuka Naval District.