Riverhead is a census-designated place roughly corresponding to the hamlet by the same name located in the Town of Riverhead in Suffolk County, on Long Island, in New York, United States. The CDP's population was 13,299 at the 2010 census. Situated at the mouth of the Peconic River, which empties into Peconic Bay where the North and South Forks of Long Island split, the town of Riverhead is the official county seat of Suffolk County.