Killingly is a town in Windham County, Connecticut, United States. The population was 17,370 at the 2010 census. It consists of the borough of Danielson and the villages of Attawaugan, Ballouville, Dayville, East Killingly, Rogers, and South Killingly.

According to the United States Census Bureau, the town has a total area of 50.0 square miles (129 km²), of which, 48.5 square miles (126 km²) of it is land and 1.5 square miles (3.9 km²) of it (2.94%) is water.

As of the census of 2010, there were 17,370 people, 6,749 households, and 4,528 families residing in the town. The population density was 358.1 people per square mile (137.9/km²). There were 7,592 housing units at an average density of 156.5 per square mile (60.3/km²). The racial makeup of the town was 93.1% White, 1.5% African American, 0.4% Native American, 1.8% Asian, 0.7% from other races, and 2.4% from two or more races. Hispanic or Latino of any race were 3.0% of the population.

The borough of Danielson, as well as the town of Killingly, is also home to a small but significant Laotian community. Both Danielson and Killingly are on the nation's list of top 50 cities with the highest percentage of citizens claiming Laotian ancestry.

Of the 6,749 households: 29.2% had children under the age of 18 living with them, 47.8% were married couples living together, 13.2% had a female householder with no husband present, and 32.9% were non-families. 25.1% of all households were made up of individuals and 9.9% had someone living alone who was 65 years of age or older. The average household size was 2.52 and the average family size was 2.98.

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