Nassau County, New York, United States

🇺🇸 Nassau County (/ˈnæsɔː/ NASS-aw) is an affluent inner suburban county located on Long Island, immediately to the east of New York City. Nassau is the sixth-most populous county in the State of New York, and reflecting an increase of (+4.2%) from the 2010 census. Its county seat is Mineola, while the county's largest town is Hempstead.

Situated on western Long Island, the County of Nassau borders New York City's borough of Queens to its west, and Long Island's Suffolk County to its east. It is the most densely populated and second-most populous county in the State of New York outside of New York City, with which it maintains extensive rail and highway connectivity, and is considered one of the central counties within the New York metropolitan area.

Nassau County contains two cities, three towns, 64 incorporated villages, and more than 60 unincorporated hamlets. Nassau County has a designated police department, fire commission, and elected executive and legislative bodies.

A 2012 Forbes article based on the American Community Survey reported Nassau County as the most expensive county and one of the highest income counties in the U.S., and the most affluent in New York state, with four of the nation's top ten towns by median income located in the county. As of 2023, the median home price in Nassau County is approximately US$700,000, and the Gold Coast of Nassau County features some of the world's most expensive real estate. Nassau County high school students often feature prominently as winners of the International Science and Engineering Fair and similar STEM-based academic awards. Cold Spring Harbor Laboratory in the Town of Oyster Bay; the Old Westbury campus of New York Institute of Technology; Zucker School of Medicine in the Village of Hempstead; and the Feinstein Institutes for Medical Research in Manhasset, are prominent life sciences research and academic institutions in Nassau County.