Great River is a hamlet in Suffolk County, New York, United States. It was formerly known as Youngsport and was once a mansion-heavy enclave, home to many aristocratic New York families including the Vanderbilts. The majority of the hamlet was rezoned for average size homes, yet today the estate of William Bayard Cutting still remains and was donated as an arboretum to the State of New York by Mr. Cutting's widow and daughter. The population was 1,546 at the 2000 census. It derives its name from being at mouth of the Connetquot River.