A county seat is a term for an administrative center for a county, primarily used in the United States. In the Northeast United States, the statutory term often is shire town, but colloquially county seat is the term in use there. Parts of the Canadian Maritimes also use the term shire town. In England, Wales and Ireland, the term county town is used. This term is still sometimes used colloquially in Scotland and Northern Ireland, but today neither are divided into administrative counties – instead being divided, respectively, into council areas and districts. Louisiana uses parishes instead of counties, and the administrative center is a "parish seat." Alaska is organized into "boroughs," which are large districts, and the administrative center is known as a "borough seat." Boroughs typically provide fewer local services than most counties, as the state government provides more services directly. About half of Alaska is part of the Unorganized Borough, a discontinuous region the state government administers directly. Some of Alaska's boroughs share geographical and administrative boundaries with cities; these are known as unified city-boroughs and result in some of Alaska's cities ranking among the geographically largest cities in the world. The Canadian province of Ontario, in addition to counties, also has districts and regional municipalities, which are effectively different types of counties in that they perform county government functions, albeit at limited (district) or expanded (regional municipality) levels.

United States counties, as in England and Canada, function as administrative subdivisions of a state and have no sovereign jurisdiction of their own, although some have authority to enact and enforce municipal ordinances. Counties administer state or provincial law at the local level as part of the decentralization of state/provincial authority. In many U.S. states, state government is further decentralized by dividing counties into townships, to provide local government services to residents of the county who do not live in incorporated cities or towns.

A county seat is usually, but not always, an incorporated municipality. The exceptions include, but are not limited to, the county seats of counties that have no incorporated municipalities within their borders, such as Arlington County, Virginia, and Baltimore County and Howard County, Maryland. (Ellicott City, the county seat of Howard County, is the largest unincorporated county seat in the U.S., followed by Towson, the county seat of Baltimore County.) The county courthouse and county administration are usually located in the county seat, but some functions may also be conducted in other parts of the county, especially if it is geographically large.