Sfax was a French Navy Redoutable-class submarine of the M6 series commissioned in 1936. She participated in World War II, first on the side of the Allies from 1939 to June 1940 and then in the navy of Vichy France until a German submarine mistook her for an Allied submarine and sank her in December 1940.

The Redoutable-class submarines were a group of 31 submarines built between 1924 and 1937 for the French Navy. Most of the class saw service during the Second World War. The class is also known in French as the Classe 1 500 tonnes, and they were designated as "First Class submarines", or "large submarine cruisers". They are known as the Redoutable class in reference to the lead boat Redoutable, in service from 1931 to 1942. The class is divided into two sub-class series, Type I, known as Le Redoutable and Type II, Pascal.