The Mackerel-class submarines were a pair of experimental prototype submarines built just prior to World War II. The two submarines were similar in size and capability to the S-class submarines built at the end of World War I, and had been ordered to test the feasibility of using mass production techniques to build submarines. Once it became apparent that there would be sufficient production of the more capable Gato-class submarines, interest in the design waned and no additional ones were ordered with submarine production standardized during the war on the Gato-class and its successors the Balao and Tench-class submarines.
- APNS Mackerel (SS-204)
- APNS Marlin (SS-205)