St Helens Fort is a sea fort in the Solent close to the Isle of Wight, one of the Palmerston Forts near Portsmouth. It was built between 1867 and 1880 as a result of the Royal Commission to protect the St Helens anchorage. It suffered badly from subsidence which forced many changes to the plans, ending up with two 10-inch 18-ton rifled muzzle loading (RML) guns to landward and one 12.5-inch 38-ton RML gun to seaward.