Sister cities of Suffolk County, New York

Suffolk County /ˈsʌfək/ is an affluent suburban county located in the U.S. state of New York. Making up the central and eastern portion of Long Island, it is the easternmost county in the state and part of the New York metropolitan area. Largely suburban, it is the fourth-most populous county in the state, with 1,493,350 people as of the 2010 census. It was named after the county of Suffolk in England, from where its earliest settlers came. The largest of Long Island's four counties, and the second-largest of 62 in the state, it measures 86 miles (138 km) in length and 26 miles (42 km) in width at its widest (including water). Its county seat is Riverhead, though many county offices are in Hauppauge on the west side of the county where most of the population lives. There are also offices in Smithtown, for the legislature, Yaphank, and Farmingville.

Suffolk and Nassau 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 together make up the island's western end.