Belleville is a city located in St. Clair County Illinois. It is also the county seat of St. Clair County. With a 2020 population of 40,009, it is the 40th largest city in Illinois and the 983rd largest city in the United States . Belleville is currently declining at a rate of -1.10% annually and its population has decreased by -10.05% since the most recent census, which recorded a population of 44,478 in 2010. Belleville reached it's highest population of 44,288 in 2010. Spanning over 23 miles, Belleville has a population density of 1,723 people per square mile.
The average household income in Belleville is $62,548 with a poverty rate of 14.10%. The median rental costs in recent years comes to $779 per month, and the median house value is $93,700. The median age in Belleville is 36.9 years, 36 years for males, and 38.4 years for females.