New Castle is a town in Westchester County, New York, United States. The population was 18,311 in the 2020 United States census, an increase over 17,569 at the 2010 census. It includes the named hamlets of Chappaqua and Millwood, but residents and businesses in the Town of New Castle can also have a designated city address of Ossining, or Millwood as well as Chappaqua or even Mt. Kisco.

Chappaqua is a hamlet and census-designated place in the town of New Castle, in northern Westchester County, New York. It is approximately 30 miles (50 km) north of New York City. The hamlet is served by the Chappaqua station of the Metro-North Railroad's Harlem Line. In the New York State Legislature it is within the New York State Assembly's 93rd district and the New York Senate's 40th district. In Congress the village is in New York's 17th District.