county, central Maryland, U.S. It is bounded by the Patapsco River to the north, Chesapeake Bay to the east, and the Patuxent River to the west and is linked across the bay to Kent Island in Queen Anne's county by the William Preston Lane, Jr., Memorial Bridge (completed 1952). The low-lying coast is lined with tidal estuaries such as those of the Magothy, Severn, South, and West rivers.

The county was created in 1650 and named for the wife of Cecilius Calvert, 2nd Baron Baltimore. Annapolis, the Maryland state capital and county seat, has long been associated with maritime activities, especially trade and tourism; it is the home of the United States Naval Academy (founded 1845), St. John's College (Saint John's College) (chartered 1784), and the Maryland State House (built 1772–79), the nation's oldest state capitol in legislative use. The northern and northwestern regions of the county contain south suburban Baltimore and Fort George G. Meade (established 1917).

In addition to governmental activities and tourism, the county's economy rests on the manufacture of search and navigation equipment and on printing, fishing, and agriculture (particularly tobacco). Area 416 square miles (1,077 square km). Pop. (2000) 489,656; (2007 est.) 512,154.