Troy is a city in Madison County, Illinois, United States. The population was 9,888 at the 2010 census.
Troy is part of the St. Louis Metropolitan Statistical Area. Its namesake in Lincoln County, Missouri is also part of this MSA, making it (along with the two O'Fallons in Illinois and Missouri) one of the few pairs of like-named municipalities to be part of the same MSA.
Troy is located at 38°43′45″N 89°53′30″W / 38.72917°N 89.89167°W (38.729236, -89.891733).
According to the United States Census Bureau, the city has a total area of 5.4 square miles (13.9 km2), of which 5.3 square miles (13.7 km2) is land and 0.08 square miles (0.2 km2), or 1.11%, is water.
At the 2000 census, there were 8,524 people, 3,100 households and 2,356 families residing in the city. The population density was 2,037.6 per square mile (787.4/km²). There were 3,201 housing units at an average density of 765.2 per square mile (295.7/km²). The racial makeup of the city was 95.48% White, 1.48% African American, 0.32% Native American, 0.70% Asian, 0.38% from other races, and 1.64% from two or more races. Hispanic or Latino of any race were 1.49% of the population.
There were 3,100 households of which 45.2% had children under the age of 18 living with them, 60.1% were married couples living together, 12.5% had a female householder with no husband present, and 24.0% were non-families. 19.5% of all households were made up of individuals and 6.8% had someone living alone who was 65 years of age or older. The average household size was 2.75 and the average family size was 3.16.
Age distribution was 30.2% under the age of 18, 8.6% from 18 to 24, 34.3% from 25 to 44, 19.2% from 45 to 64, and 7.7% who were 65 years of age or older. The median age was 33 years. For every 100 females there were 96.0 males. For every 100 females age 18 and over, there were 91.5 males.
The median household income was $53,720, and the median family income was $59,643. Males had a median income of $41,705 versus $27,542 for females. The per capita income for the city was $21,174. About 2.1% of families and 3.4% of the population were below the poverty line, including 3.4% of those under age 18 and 4.5% of those age 65 or over.