Fort Pierce, also spelled Ft. Pierce, is a city in St. Lucie County, Florida, USA. It is known as The Sunrise City.It is the county seat of St. Lucie County. Fort Pierce is part of the Port St. Lucie, Florida Metropolitan Statistical Area . According to the United States Census Bureau, the city has a total area of 20.8 square miles (54square kilometers). 14.7 square miles (38square kilometers) of it is land and 6.0 square miles (16square kilometers) of it (35.00percent) is water.