county, northeastern Maryland, U.S., lying at the head of Chesapeake Bay and bounded by Pennsylvania to the north, Delaware to the east, the Sassafras River to the south, and the Susquehanna River to the west. The county is drained by Octoraro Creek, the Northeast River, and the Elk River, which is the western terminus of the Chesapeake and Delaware Canal (completed 1829). Parklands include Elk Neck Demonstration Forest and Elk Neck State Park. The county was formed in 1674 and named for Cecilius Calvert, 2nd Baron Baltimore, the founder of Maryland. The county seat is Elkton.

Principal economic activities are agriculture, mining, and light manufacturing. Area 348 square miles (902 square km). Pop. (2000) 85,951; (2007 est.) 99,695.