county, central Maryland, U.S., bordered by the South Branch Patapsco River to the north, the Patapsco River to the northeast, and the Patuxent River to the west and southwest. The county is bracketed by Patuxent River State Park in the west and Patapsco Valley State Park in the east.

Howard county was created in 1851, having earlier been (from 1838) a district of Anne Arundel county. It was named for statesman and Revolutionary War hero John Eager Howard. The county seat, Ellicott City (formerly Ellicott's Mills), became the first railroad terminus in the United States (1830) as part of the Baltimore and Ohio Railroad. The planned community of Columbia was founded in the mid-1960s.

The county's economy is based on research and development, wholesale trade, and manufacturing. Area 252 square miles (653 square km). Pop. (2000) 247,842; (2007 est.) 273,669.