Nassau County is a suburban county in the New York Metropolitan Area east of New York City in the U.S. state of New York. As of the 2000 census, the population was 1,334,544. The name of the county comes from an old name for Long Island, which was at one time named Nassau, after William of Nassau, Prince of Orange (who later became King William III of England.) The county colors, orange and blue, are also the colors of the House of Orange. Nassau County's county seat is Mineola.

Nassau and Suffolk counties together are generally referred to as "Long Island" by area residents — as distinct from the New York City boroughs of Queens (Queens County) and Brooklyn (Kings County), which physically make up the island's westernmost end.

In 2005, Forbes magazine named Nassau County, along with Suffolk County, New York, as the safest region in the United States, with the lowest crime rate.

As of 2008, Nassau County is the second richest county per capita in the State of New York and the 30th richest in the nation, with a per capita income of $32,894. As of 2005, Nassau had the second highest median property tax in the nation at $7,025.