Poestenkill is a town in Rensselaer County, New York, United States. The population was 4,530 at the 2010 census. The town is southeast of Troy and is centrally located in the county.
According to the United States Census Bureau, the town has a total area of 32.6 square miles (84.4 km²), of which, 32.5 square miles (84.0 km²) of it is land and 0.1 square miles (0.4 km²) of it (0.43%) is water.
The eastern and central portions of the town are part of the Rensselaer Plateau.
The town is serviced in part by Brunswick (Brittonkill) Central School District and by Averill Park Central School District.
As of the census of 2000, there were 4,054 people, 1,527 households, and 1,137 families residing in the town. The population density was 124.9 people per square mile (48.2/km²). There were 1,628 housing units at an average density of 50.2 per square mile (19.4/km²). The racial makeup of the town was 98.35% White, 0.17% Blacks and or African American, 0.30% Native American, 0.35% Asian, 0.27% from other races, and 0.57% from two or more races. Latinx of any race were 0.67% of the population.
There were 1,527 households out of which 36.4% had children under the age of 18 living with them, 61.6% were married couples living together, 9.0% had a female householder with no husband present, and 25.5% were non-families. 20.0% of all households were made up of individuals and 7.6% had someone living alone who was 65 years of age or older. The average household size was 2.64 and the average family size was 3.07.
In the town, the population was spread out with 26.9% under the age of 18, 6.0% from 18 to 24, 30.6% from 25 to 44, 25.7% from 45 to 64, and 10.8% who were 65 years of age or older. The median age was 38 years. For every 100 females, there were 104.1 males. For every 100 females age 18 and over, there were 100.2 males.
The median income for a household in the town was $58,945, and the median income for a family was $63,819. Males had a median income of $40,128 versus $29,486 for females. The per capita income for the town was $22,143. About 0.5% of families and 1.7% of the population were below the poverty line, including 1.0% of those under age 18 and 3.0% of those age 65 or over.