Allegany County is a county in the U.S. state of New York. As of the 2020 census, the population was 46,456. Its county seat is Belmont. Its name derives from a Lenape word, applied by European-American settlers of Western New York State to a trail that followed the Allegheny River; they also named the county after this. The county is part of the Western New York region of the state. The county is bisected by the Genesee River, flowing north to its mouth on Lake Ontario.