county, southern Maryland, U.S. It consists of a tidewater peninsula bordered by the Patuxent River to the northeast, Chesapeake Bay to the east, the Potomac River to the south, and the Wicomico River to the west. Landmarks include the Patuxent Naval Air Test Center (opened 1941) and Saint Mary's River and Point Lookout state parks.

The oldest county in Maryland, St. Mary's (formed 1637) contains St. Clement's Island (Saint Clements Island), the site of the first landing (March 25, 1634) by English settlers under the Maryland Charter granted to Cecilius Calvert, 2nd Baron Baltimore. It was named for Mary, the mother of Jesus. St. Mary's City (Saint Marys City) was Maryland's first European settlement, first provincial capital (until 1694), and first county seat (1637–1710).

The present county seat is Leonardtown. Tobacco farming, lumbering, fishing, and technological services are the main economic activities. Area 361 square miles (936 square km). Pop. (2000) 86,211; (2007 est.) 100,378.