Yukon–Koyukuk Census Area is a census area in the U.S. state of Alaska. As of the 2020 census, the population was 5,343, down from 5,588 in 2010. With an area of 147,842.51 sq mi (382,910.3 km2), it is the largest of any county or county-equivalent in the United States, or slightly larger than the entire state of Montana. It is part of the unorganized borough of Alaska and therefore has no borough seat. Its largest communities are the cities of Galena, in the west, and Fort Yukon, in the northeast.

In the United States, a county or county equivalent is an administrative or political subdivision of a U.S. state or other territories of the United States which consists of a geographic area with specific boundaries and usually 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. The specific governmental powers of counties 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; New York City is uniquely partitioned into five counties, referred to at the city government level as boroughs. 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, those counties in Connecticut, Rhode Island, eight of Massachusetts's 14 counties, and Alaska's Unorganized Borough have no government power, existing only as geographic distinctions.