SSG is an obsolete hull classification symbol in the United States Navy. It was applied to the Regulus missile-launching submarines from the 1950s. Only four submarines were designated SSG: USS Tunny, USS Barbero, , and . Tunny and Barbero were modified World War II s, while Grayback and Growler were custom made launch platforms. A fifth Regulus boat, , was built with nuclear power. These ships were redesignated with the 1964 removal of the Regulus missile from service.
The Soviet Union built the s to carry nuclear cruise missiles to attack the United States, but these were later converted to carry anti shipping cruise missiles. Three variants of the s, called the "Single Cylinder", "Twin Cylinder", and "Long Bin" were made.