Flat is a census-designated place (CDP) in Yukon-Koyukuk Census Area, Alaska, United States. As of the 2010 Census, the population of the CDP was 0, down from 4 residents in 2000. Its post office closed in January 2004. Summer residents visited less and none remained by 2014. As of 2023, few buildings are inhabitable with hundreds of collapsed ruins.

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.