Belleville is a city in and the county seat of St. Clair County, Illinois, United States. The population was 42,404 at the 2020 census, making it the most populated city in the Metro East region of Greater St. Louis, and in all of Southern Illinois. Due to its proximity to Scott Air Force Base, the city has a significant population of military and federal civilian personnel. It is the seat of the Roman Catholic Diocese of Belleville and home to the National Shrine of Our Lady of the Snows.

George Blair named the city of Belleville in 1814, after the French phrase belle ville, meaning "beautiful city". Because Blair donated an acre of his land for the town square and an additional 25 acres (10 ha) adjoining the square for the new county seat, the legislature transferred the county seat from the village of Cahokia. The latter had been established by French colonists as a mission village in the late 17th century.

Belleville was incorporated as a village in 1819, and became a city in 1850. Major immigration in the mid-19th century to this area occurred following revolutions in Germany, and most of the European-American population is of German ancestry. Many of the educated Germans fled their homeland after the failure of the German Revolution in 1848. Belleville was the center of the first important German settlement in Illinois. By 1870, an estimated 90% of the city's population was either German-born or of German descent.

After the Civil War, Belleville became a manufacturing center producing nails, printing presses, gray iron castings, agricultural equipment, and stoves. Belleville became known as "The Stove Capital of the World." The first brewery in Illinois was established in Belleville. In 1868, Gustav Goelitz founded the candy company that is known today as "Jelly Belly."

An immense deposit (400,000 acres (1,600 km2)) of bituminous coal was found in St. Clair County. By 1874, some farmers had become coal miners. One hundred shaft mines were in operation in and around Belleville. The coal brought the steam railroad to town, which allowed for the transport of many tons of coal to be shipped daily from Belleville to St. Louis on the west side of the Mississippi River, for use in its industries, homes and businesses. Later, Belleville had the first electric trolley in the state.

The first style of houses in Belleville were simple brick cottages, known locally as "German street houses" or "row houses." Architectural styles flourished in greater variety, featuring American Foursquare, French Second Empire, Greek Revival, Gothic Revival, Italianate, Queen Anne, and Victorian. The Belleville Historic District, which was listed on the National Register of Historic Places in 1976, comprises 73 contributing properties.

The "Old Belleville Historic District," was defined and recognized in 1974 and is the city's first historic district. The city also has designated two more historic districts: "Hexenbuckel" (est. in 1991) and "Oakland" (est. in 1995).

Belleville's early German immigrants were educated, with most of them having graduated from German universities. They were nicknamed "Latin Farmers" because of this. After 1836 Gustav Koerner contributed to establish the city's public library. The Belleville Public Library is the state's oldest, predating the Illinois State Library by three years. The German settlers also founded choral and dramatic groups, as well as literary societies. They established one of the first kindergartens in the country here.

In 1990, as reported by the Belleville News-Democrat using the US Census data, the city surpassed the neighboring city of East St. Louis to become the most populated city in the Metro East and Southern Illinois.

The National Civic League recognized Belleville in 2011 as one of the ten recipients of the All-America City Award. Belleville is coterminous with the now defunct Belleville Township.

In 2021, residents of the Illinois city elected Patty Gregory as Belleville's first female Mayor.

Belleville is located at 38°31′18″N 89°59′43″W / 38.52167°N 89.99528°W / 38.52167; -89.99528 (38.521567, −89.995208).

According to the 2010 census, Belleville has a total area of 23.49 square miles (60.84 km2), of which 22.74 square miles (58.90 km2) (or 98.83%) is land and 0.269 square miles (0.70 km2) (or 1.17%) is water. Richland Creek flows through much of Belleville.

At the 2000 census there were 41,410 people, 17,603 households, and 10,420 families living in the city. The population density was 2,196.4 inhabitants per square mile (848.0/km2). There were 19,142 housing units at an average density of 1,015.3 per square mile (392.0/km2). The racial makeup of the city was 81.51% White, 15.51% African American, 0.26% Native American, 0.81% Asian, 0.07% Pacific Islander, 0.41% from other races, and 1.43% from two or more races. Hispanic or Latino of any race were 1.63%.

Of the 17,603 households 28.4% had children under the age of 18 living with them, 42.0% were married couples living together, 13.5% had a female householder with no husband present, and 40.8% were non-families. 35.1% of households were one person and 14.0% were one person aged 65 or older. The average household size was 2.27 and the average family size was 2.95.

The age distribution was 23.4% under the age of 18, 9.0% from 18 to 24, 30.3% from 25 to 44, 20.1% from 45 to 64, and 17.2% 65 or older. The median age was 37 years. For every 100 females, there were 88.9 males. For every 100 females age 18 and over, there were 85.0 males.

The median household income was $35,979 and the median family income was $46,426. Males had a median income of $33,361 versus $25,375 for females. The per capita income for the city was $18,990. About 9.3% of families and 11.7% of the population were below the poverty line, including 16.2% of those under age 18 and 9.3% of those age 65 or over.

As of the 2010 Census, there were 44,478 people, 18,795 households, 11,081 families living in the city. The population density was. The racial makeup of the city was 69.8% White, 25.4% African American, 0.3% Native American, 1.0% Asian, 0.1% Pacific Islander, 0.6% from other races, and 2.8% from two or more races. Hispanic or Latino of any race were 2.6% of the population

Of the 18,795 households 27.7% had children under the age of 18 living with them, 38% were married couples living together, 16.4 had a female householder with no man present, and 41% were non-families. 34.4% of households were one person and 10.8% had someone living alone who was 65 years of age of older. The average household size was 2.3 and the average family size was 2.9.

The age distribution was 23.3% under the age of 18, and 12.9% over the age of 65.

The Belleville Philharmonic Society was formed in 1866, making it the second oldest philharmonic orchestra in the country. With the increase in the black population and migrants from the South, musicians developed who played blues and jazz; later rock clubs were added to the scene.

Jay Farrar (now of Son Volt), Mike Heidorn, and Jeff Tweedy (now of Wilco) of the now-defunct alt country group Uncle Tupelo are from Belleville. Another major musician was Neal Doughty, keyboardist for 1970s rock band REO Speedwagon.

Belleville Township High School District 201 is the public high school district.

Within Belleville are four high schools; namely, the public Belleville High School-East and Belleville High School-West (of the high school district), and the private Althoff Catholic High School (of the Roman Catholic Diocese of Belleville), and Governor French Academy.

In addition, small portions of the city are within these districts: East St. Louis School District 189 (which operates East St. Louis Senior High School), Mascoutah Community Unit School District 19 (which operates Mascoutah Community High School), and Freeburg Community High School District 77 (which operates Freeburg Community High School).

Belleville is also home to numerous grade school districts, including Belleville School District 118, Belle Valley School District 119, Harmony Emge School District 175, Signal Hill School District 181, Whiteside School District 115, and Freeburg Community Consolidated School District 70.

Parochial grade schools include St. Peter's Cathedral, St. Augustine of Canterbury Church, St. Teresa, Blessed Sacrament, Our Lady Queen of Peace, and Zion Lutheran School. The Catholic schools belong to the diocese.

Higher education in Belleville includes Southwestern Illinois College, a public community college.

The city was formerly home to Lindenwood University – Belleville from 2009 to 2020.

The Belleville News-Democrat, is the city's daily newspaper. The News-Democrat is part of the McClatchy chain and covers the Metro East region and Southwestern Illinois. The city is also served by the St. Louis Post-Dispatch, the area's major metropolitan daily. Belleville receives the signals of most radio and TV stations based in St. Louis.

Belleville has three St. Louis MetroLink stations connecting it to St. Louis and the network via light rail: Memorial Hospital, Belleville, and College.

Illinois State Highways 15, 158, 159, 177, 13 and 161 all pass through Belleville. Belleville is also adjacent to Interstate highways 64 and 255. I-64 is an east–west highway extending from Wentzville, Missouri to Virginia Beach, Virginia and is the major route from Belleville to downtown St. Louis. I-255 is part of a system of expressways that together form a loop around St. Louis.

Belleville has a bicycle trail that runs through the city from Southside Park to Southwestern Illinois College and Scott Air Force Base; it is mainly used for recreational purposes.

Belleville's area airports are Scott Air Force Base and MidAmerica St. Louis Airport, which is served by Allegiant Air. The nearest major airport with regularly scheduled commercial passenger service is St. Louis Lambert International Airport.

Bethany Place in Belleville provides services for those with HIV/AIDS.