Sullivan County is a county in the U.S. state of New York. As of the 2020 census, the population was 78,624. The county seat is Monticello. The county's name honors Major General John Sullivan, who was labeled at the time as a hero in the American Revolutionary War in part due to his successful campaign against the Iroquois. The county is part of the Hudson Valley region of the state.

Monticello is a village located in Thompson, Sullivan County, within the Catskills region of New York, United States. It is the seat for the town of Thompson, and the county seat of Sullivan County. The population was 7,173 at the 2020 census. The village was named after the residence of Thomas Jefferson.