Belleville, IL has a population of 42,729 and is the 960th largest city in the United States. The population density is 1,846 per sq mi which is 731% higher than the Illinois average and 1938% higher than the national average. The median age in Belleville is 37 which is approximately 1% lower than the Illinois average of 37. In Belleville, 48% of the population over 15 years of age are married, 97% speak English and 1% speak Spanish. 65% of Belleville residents were born in Illinois, 32% were born out of state, 1% were born outside of the United States and 2% were foreign born.