The Grayback-class submarine was a class of two guided missile-carrying submarines of the United States Navy. They carried the Regulus I and Regulus II nuclear cruise missiles, deployed 1957–64, that were rapidly phased out by Polaris Submarine Launched Ballistic Missiles (SLBMs). They and USS Halibut were the sole submarines designed specifically to carry Regulus missiles, and the only submarines capable of carrying Regulus II. However, USS Tunny and USS Barbero were modified earlier to carry two Regulus I missiles per boat.