Plessis is a hamlet and census-designated place in the town of Alexandria in Jefferson County, New York, United States. As of the 2020 census, Plessis had a population of 126. Plessis had a post office from September 16, 1823, until March 26, 2011; it still has its own ZIP code, 13675. New York State Route 26 passes through the community.
According to the U.S. Census Bureau, the community has an area of , all land.
The school district is Alexandria Central School District.