Petersburg, IL has a population of 2,333 and is the 10,187th largest city in the United States. The population density is 1,493 per sq mi which is 572% higher than the Illinois average and 1548% higher than the national average. The median age in Petersburg is 35 which is approximately 7% lower than the Illinois average of 37. In Petersburg, 58% of the population over 15 years of age are married, 99% speak English and 1% speak Spanish. 81% of Petersburg residents were born in Illinois, 17% were born out of state, 0% were born outside of the United States and 1% were foreign born.