Calvert County is a county located in the U.S. state of Maryland. As of the 2020 census, the population was 92,783. Its county seat is Prince Frederick. The county's name is derived from the family name of the Barons of Baltimore, the proprietors of the English Colony of Maryland. Calvert County is included in the Washington–Arlington–Alexandria, DC–VA–MD–WV Metropolitan Statistical Area. It occupies the Calvert Peninsula, which is bordered on the east by Chesapeake Bay and on the west by the Patuxent River. Calvert County is part of the Southern Maryland region. The county has one of the highest median household incomes in the United States. It is one of the older counties in Maryland, after St. Mary's, Kent County and Anne Arundel counties.

Prince Frederick is an unincorporated community and census-designated place (CDP) in Calvert County, Maryland, United States. As of the 2020 census, the population of Prince Frederick was 3,226, up from 2,538 in 2010. It is the county seat of Calvert County.