Watertown is a town in Litchfield County, Connecticut, United States. The population was 22,105 at the 2020 census. The ZIP codes for Watertown are 06795 (for most of the town) and 06779 (for the Oakville section). It is a suburb of Waterbury. The urban center of the town is the Watertown census-designated place, with a population of 3,938 at the 2020 census.

Colonization of the area today called Watertown began around 1657. In that time, the colony was called “Mattatock”, though it had several variations in spelling through the years. The land where Watertown is now located, having originally belonged to Mattatock, officially changed its name to Watterbury (now Waterbury) by record on March 20, 1695, by consensus of a council. The original Colony of Mattatuck, which became Watterbury, then Waterbury in name, comprised a much greater land area than Waterbury does today. Thomas Judd and other families were among the first investors to buy the land as a group. The Town of Watertown was officially incorporated in 1780.

Watertown is in southeastern Litchfield County and is bordered to the southeast by the city of Waterbury in New Haven County. Other bordering towns are Middlebury to the south, Woodbury and Bethlehem to the west, Morris to the northwest, and Thomaston to the east. It is in the Eastern Standard time zone. The elevation at the town center is 583 feet (178 m).

According to the United States Census Bureau, the town has a total area of 29.5 square miles (76.4 km), of which 29.0 square miles (75.1 km²) are land and 0.50 square miles (1.3 km), or 1.72%, are water. Oakville, which is often mistaken for a separate town, is in the southeast part of Watertown. Although Oakville has its own post office and ZIP code, it does not have a charter or town government of its own. Oakville receives all of its town services (police, fire, water and so on) from Watertown.

As of the census of 2000, there were 21,661 people, 8,046 households, and 5,994 families residing in the town. The population density was 743.0 inhabitants per square mile (286.9/km²). There were 8,298 housing units at an average density of 284.6 per square mile (109.9/km). The racial makeup of the town was 96.46% White, 0.75% African American, 0.12% Native American, 1.27% Asian, 0.05% Pacific Islander, 0.48% from other races, and 0.87% from two or more races. Hispanic or Latino of any race were 1.87% of the population.

There were 8,046 households, out of which 34.7% had children under the age of 18 living with them, 61.7% were married couples living together, 9.4% had a female householder with no husband present, and 25.5% were non-families. 21.7% of all households were made up of individuals, and 9.4% had someone living alone who was 65 years of age or older. The average household size was 2.67 and the average family size was 3.13.