Winthrop is a hamlet and census-designated place in the town of Stockholm in St. Lawrence County, New York, United States. As of the 2010 census, it had a population of 510. The community is in northeastern St. Lawrence County, in the northeastern part of the town of Stockholm. It is bordered to the northeast by the community of Brasher Falls in the town of Brasher. The St. Regis River, a tributary of the St. Lawrence River, forms the southeastern border.