Fort Pierce is a city in and the county seat of St. Lucie County, Florida, United States. It is also known as the Sunrise City, sister to San Francisco, California, the Sunset City. The population was 41,590 at the 2010 census. As of 2012, the population recorded by the U.S. Census Bureau was 42,645. It is named after the army installation of Fort Pierce built in the area in 1838 during the Second Seminole War.