Frederick County is located in Maryland, United States. As of the 2020 U.S. census, the population was 271,717. The county seat is Frederick. The county is part of the Capital region of the state.

Like other outlying sections of the Washington metropolitan area, Frederick County has experienced a rapid population increase since the 1980s. It borders the southern border of Pennsylvania and the northeastern border of Virginia.

Catoctin Mountain Park in the county is the location of Camp David, a U.S. presidential retreat, and Fort Detrick, a U.S. Army base.

The namesake of Frederick County and its county seat is unknown, but it was probably either Frederick, Prince of Wales, or Frederick Calvert, 6th Baron Baltimore.

Frederick County was created in 1748 by the Province of Maryland from parts of Prince George's County and Baltimore County.

In 1776, following US independence, Frederick County was divided into three parts. The westernmost portion became Washington County, named after George Washington, the southernmost portion became Montgomery County, named after another Revolutionary War general, Richard Montgomery. The northern portion remained Frederick County.

In 1837, a part of Frederick County was combined with a part of Baltimore County to form Carroll County which is east of current day Frederick County.

The county has a number of properties on the National Register of Historic Places.