St. Lucie County is a county located in the southeastern portion of the U.S. state of Florida. As of the 2020 census, the population was 329,226. The county's seat is Fort Pierce. St. Lucie County is included in the Port St. Lucie, FL Metropolitan Statistical Area.

The area was originally inhabited by the Ais tribe, a hunter-gatherer culture whose territory extended from south of the St. John's river to the St. Lucie Inlet.

The area was given several names by the Spanish including Rio de Ays (later changed to Indian River) as well as Santa Lucia, named after the short-lived late 16th-century Spanish fort that bore its name farther south. The fabled 1715 Spanish treasure fleet sank off the area that is now St. Lucie County, leading to the regional naming of the area as the Treasure Coast.

During the early 19th century, the Spanish government issued several land grants in the area, one of which went to settler James Hutchinson. The grant contained 2,000 acres
(8.1 km2) and today the barrier island Hutchinson Island still retains his name.