Fairfax is a town in Franklin County, Vermont, United States. The population was 4,285 at the 2010 census, up from 3,765 at the 2000 census.
The town of Fairfax is in southern Franklin County and is bordered by Chittenden County to the south. According to the United States Census Bureau, the town has a total area of 40.4 square miles (104.7 km2), of which 39.8 square miles (103.2 km2) is land and 0.58 square miles (1.5 km2), or 1.44%, is water. The Lamoille River, a tributary of Lake Champlain, flows from east to west across the southern part of the town, passing through the communities of Fairfax and Fairfax Falls.
As of the census of 2010, there were 4,285 people, 1,591 households, and 1,226 families residing in the town. The population density was 105.8 people per square mile (40.8/km2). There were 1,683 housing units at an average density of 41.6 per square mile (16.0/km2). The racial makeup of the town was 97.8% White and 1.3% Hispanic or Latino. There were 39.8% of households under the age of 18 living with them, 64.2% were married couples living together, 8.2% had a female householder with no husband present, and 22.9% were non-families and 16.0% of all households were made up of individuals and 16.2% had someone living alone who was 65 years of age or older.