county, south-central Maryland, U.S., consisting of a tidewater peninsula lying between the Patuxent River to the west and south and Chesapeake Bay to the east. Calvert Cliffs State Park towers over the bay, exposing fossils from the Miocene Epoch that are 15 to 20 million years old. The county was created in 1654. Originally called Patuxent county, it was renamed for Cecilius Calvert, 2nd Baron Baltimore (Baltimore, George Calvert, 1st Baron), who founded Maryland as a sanctuary for Roman Catholics. Prince Frederick is the county seat. Calvert has the smallest area of any county in the state.

Tobacco farming, tourism, and fishing are the principal economic activities. Area 215 square miles (557 square km). Pop. (2000) 74,563; (2007 est.) 88,223.