A merchant submarine is a type of submarine intended for trade, and being without armaments, it is not considered a warship like most other types of submarines. The intended use would be blockade running, or to dive under Arctic ice.

The Liuzzi class was a class of four submarines built by Tosi in Taranto for the Royal Italian Navy. The submarines were built in 1939 and began their Second World War service in the Mediterranean Sea, where Liuzzi was sunk. The three surviving boats were transferred to the BETASOM Atlantic submarine base at Bordeaux in 1940. After Tarantini was sunk, Bagnolini and Giuliani were selected for conversion to "transport submarines" in order to exchange rare or irreplaceable trade goods with Japan. Cargo capacity of 160 tons reduced reserve buoyancy from between 20 and 25% to between 3.5 and 6%; and armament was reduced to defensive machine guns. The submarines saw action in the Second World War during which they collectively sunk 5 freighters and 1 light cruiser and were eventually either sunk or captured.