In the United States, a county or county equivalent is an administrative subdivision of a state or territory, typically with defined geographic boundaries and some level of governmental authority. The term "county" is used in 48 states, while Louisiana and Alaska have functionally equivalent subdivisions called parishes and boroughs, respectively. Counties and other local governments exist as a matter of U.S. state law, so the specific governmental powers of counties may vary widely between the states, with many providing some level of services to civil townships, municipalities, and unincorporated areas. Certain municipalities are in multiple counties. Some municipalities have been consolidated with their county government to form consolidated city-counties or have been legally separated from counties altogether to form independent cities. Conversely, counties in Connecticut and Rhode Island, eight of Massachusetts's 14 counties, and Alaska's Unorganized Borough have no government power, existing only as geographic distinctions.

The United States Census Bureau uses the term "county equivalent" to describe places that are comparable to counties, but called by different names. Louisiana parishes, the organized boroughs of Alaska, independent cities, and the District of Columbia are equivalent to counties for administrative purposes. Alaska's Unorganized Borough is further divided into 11 census areas that are statistically equivalent to counties. In 2024, the U.S. Census Bureau began to also recognize Connecticut's councils of governments, which took over some of the regional powers from the state's former county governments, as county equivalents.

Territories of the United States do not have counties; instead, the United States Census Bureau also divides them into county equivalents. The U.S. Census Bureau counts American Samoa's districts and atolls as county equivalents. American Samoa locally has places called "counties", but these entities are considered to be "minor civil divisions" (not true counties) by the U.S. Census Bureau.

The number of counties per state ranges from the three counties of Delaware to the 254 counties of Texas. County populations also vary widely; in 2017, according to the Census Bureau, more than half the U.S. population was concentrated in just 143 of the more than 3,000 counties, or just 4.6% of all counties. The five most populous counties, ordered from most to least, are Los Angeles County, California; Cook County, Illinois; Harris County, Texas; Maricopa County, Arizona; and San Diego County, California.

As of 2022^[update], there are 3,144 counties and county-equivalents in the 50 states and the District of Columbia. If the 100 county equivalents in the U.S. territories are counted, then the total is 3,244 counties and county-equivalents in the United States.

The idea of counties originated with the counties of England. English (after 1707, British) colonists brought to their colonies in North America a political subdivision that they already used in the British metropole: the counties. Counties were among the earliest units of local government established in the Thirteen Colonies that would become the United States. Virginia created the first counties in order to ease the administrative workload in Jamestown. The House of Burgesses divided the colony first into four "incorporations" in 1617 and finally into eight shires (or counties) in 1634: James City, Henrico, Charles City, Charles River, Warrosquyoake, Accomac, Elizabeth City, and Warwick River. America's oldest intact county court records can be found at Eastville, Virginia, in Northampton (originally Accomac) County, dating to 1632. Maryland established its first county, St. Mary's in 1637. In 1639, the Province of Maine founded York County. Massachusetts followed in 1643. Pennsylvania and New York delegated significant power and responsibility from the colony government to county governments and thereby established a pattern for most of the United States, although counties remained relatively weak in New England.

When independence came, the framers of the Constitution left the matter to the states. Subsequently, state constitutions conceptualized county governments as arms of the state. Louisiana instead adopted the local divisions called parishes that dated back to both the Spanish colonial and French colonial periods when the land was dominated by the Catholic Church. In the twentieth century, the role of local governments strengthened and counties began providing more services, acquiring home rule and county commissions to pass local ordinances pertaining to their unincorporated areas. In 1955, delegates to the Alaska Constitutional Convention wanted to avoid the traditional county system and adopted their own unique model with different types of boroughs varying in powers and duties.

In some states, these powers are partly or mostly devolved to the counties' smaller divisions usually called townships, though in New York, New England and Wisconsin they are called "towns". The county may or may not be able to override its townships on certain matters, depending on state law.