Westchester County is located in the U.S. state of New York. It is the seventh most populous county in the State of New York and the most populous north of New York City. According to the 2020 United States Census, the county had a population of 1,004,456, its highest decennial count ever and an increase of 55,344 (5.8%) from the 949,113 counted in 2010. Westchester covers an area of 450 square miles (1,200 km2), consisting of six cities, 19 towns, and 23 villages. Established in 1683, Westchester was named after the city of Chester, England. The county seat is the city of White Plains, while the most populous municipality in the county is the city of Yonkers, with 211,569 residents per the 2020 census. The county is part of the Hudson Valley region of the state.

The Hudson Valley comprises the valley of the Hudson River and its adjacent communities in the U.S. state of New York. The region stretches from the Capital District including Albany and Troy south to Yonkers in Westchester County, bordering New York City.