Frederick County is a county located in the western part of the state of Maryland, bordering the southern border of Pennsylvania and the northeastern border of Virginia. It is a part of the Washington-Baltimore Metropolitan Area, and is often recognized as part of Western Maryland. The county is home to Catoctin Mountain Park and the presidential retreat Camp David. The county seat is Frederick, which was home to several celebrated historical figures like Francis Scott Key. The county was named for Frederick Calvert, 6th Baron Baltimore.

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

In 1776, Frederick County was divided into three parts. The westernmost portion became Washington County, named after George Washington, the easternmost portion became Montgomery County, named after another Revolutionary War general, Richard Montgomery. The central portion remained Frederick County.

In 1837 a part of Frederick County was combined with a part of Baltimore County to form Carroll County.
According to the U.S. Census Bureau, the county has a total area of 1,728 km² (667 mi²). 1,717 km² (663 mi²) of it is land and 12 km² (4 mi²) of it is water. The total area is 0.67% water.

Attractions in Frederick include the Clustered Spires, a monument to Francis Scott Key, the National Museum of Civil War Medicine, Monocacy battlefield and South Mountain battlefields, and the Schifferstadt Architectural Museum.