Fort Mansfield was a coastal artillery installation located on Napatree Point, a long barrier beach in the village of Watch Hill in Westerly, Rhode Island, United States.

Its 1884 report warned of the defenseless condition of the USA's coasts and recommended a system of fortifications to protect harbors and coastal cities from invasion.

In 1898, the United States government purchased 60 acres (240,000 m2) on Napatree Point, a long, narrow spit of land jutting out from Watch Hill, Rhode Island.

The installation could be bombarded with impunity, and an invading party could assault the fort opposed by nothing better than small-arms fire, depending on where it landed.

A report on the war games stated, "I believe I could capture Ft. Mansfield with a fleet of coal barges, equipped with 6-inch rapid fire guns".

During World War II, a battery of two 16"/50 caliber Mark 2 guns was proposed for the Watch Hill area at the Oak's Inn Military Reservation in Misquamicut, Rhode Island, but it was never built.

[5][6] In August 2009, a small group of young adults interested in historical preservation began a cleanup project on Fort Mansfield.