Constantia is a town in Oswego County, New York, United States. The population was 4,973 at the 2010 census.
The Town of Constantia is in the southeast part of the county. Located with the town is a hamlet and census-designated place also named Constantia.
The town was first settled around 1793 and was known then as "Rotterdam." Francis Adrian Vanderkemp and Marc Isambard Brunel were two of its citizens. The Town of Constantia was created from part of the Town of Mexico in 1808. In 1825, the town was reduced by the formation of the Town of Hastings, and was reduced again in 1839 to form the Town of West Monroe.
According to the United States Census Bureau, the town has a total area of 99.6 square miles (258.0 km2), of which, 56.9 square miles (147.3 km2) of it is land and 42.8 square miles (110.8 km2) of it (42.93%) is water.
The south town line is the opposite shore of Oneida Lake, which is the border of Madison County, New York. The east town line is the border of Oneida County, New York.
The Erie Canal, runs through the town, using Oneida Lake as a waterway.