Fort Pierce is a city in and the county seat of St. Lucie County, Florida, United States. The city is part of the Treasure Coast region of Atlanti…Fort Pierce is a city in and the county seat of St. Lucie County, Florida, United States. The city is part of the Treasure Coast region of Atlantic Coast Florida. It is also known as the Sunrise City. Per the 2020 census, the population was 47,297.