During its first century as a city, Chicago grew at a rate that ranked among the fastest growing in the world. Within the span of forty years, the city's population grew from slightly under 30,000 to over 1 million by 1890. By the close of the 19th century, Chicago was the fifth largest city in the world. Within fifty years of the Great Chicago Fire of 1871, the population had tripled to over 3 million.
As of the 2010 United States Census, there were 2,695,598 people and 1,194,337 households residing within the city limits of Chicago. More than half the population of the state of Illinois lives in the Chicago metropolitan area. The 2000 United States Census had shown the population density of the city itself was 12,750.3 people per square mile (4,923.0/km²), making it one of the nation's most densely populated cities. There were 1,152,868 housing units at an average density of 5,075.8 per square mile (1,959.8/km²). Of the 1,061,928 households in the 2000 census, 28.9% had children under the age of 18 living with them, 35.1% were married couples living together, 18.9% had a female householder with no husband present, and 40.4% were non-families. The median income for a household in the city was $38,625 in 2000, and the median income for a family was $46,748. Males had a median income of $35,907 versus $30,536 for females. Below the poverty line were 19.6% of the population and 16.6% of the families.
While most of Chicago and its surrounding residential areas are generally regarded as being somewhat racially segregated, the city's unique culture arises from its being a melting pot, with nearly even percentages of European Americans, African Americans, and Latinos as well as sizable populations of Asians. The White, Black and Hispanic communities extend radially outward from the center of the city. The racial makeup of the city in 2010 was 45.3% white (31.7% non-Hispanic white), 32% black, 5% Asian, and 3% from two or more races. The ethnic makeup of the population is 28% Hispanic and 72% belong to non Hispanic background.
In 2000, 21.7% of the population was foreign born; of this, 56.3% came from Latin America, 23.1% from Europe, 18.0% from Asia and 2.6% from other parts of the world. Chicago has the fifth highest foreign-born population in the United States.
In 2016, the population of Hispanics exceeded that of Blacks to become Chicago's second largest minority group with non-Hispanic Whites representing 32.6% of the population, Hispanics at 29.7% of the population, and Blacks at 29.3% of the population. The large Hispanic population is rather recent, with the segregation between Hispanics and non-Hispanic Whites being low when compared to Blacks and non-Hispanic Whites. Over the years neighborhoods have seen gradual ethnic change with White ethnic neighborhoods like Brighton Park transitioning to Hispanic, while former Latino neighborhoods like West Town transition to majority non-Hispanic White.
The main ethnic groups in Chicago include Irish, German, Italian, Mexican, Assyrian, Arab, Bangladeshi, Jews, English, Bosnian, Croatian, Bulgarian, Czech, Greek, Black, Korean, Chinese, Indian, Filipino, Lithuanian, Macedonian, Montenegrin, Albanian, Pakistani, Polish, Hungarian, Romanian, Russian, Serbian, Slovak, Swedish, Ukrainian, Dutch, Belgian, Cuban, Vietnamese, and Puerto Rican. Chicago is also home to 30,000 natives of Iran (mostly Azeri).
Chicago has a large Irish American population, with many still residing on the South Side. The early years of Chicago coincided with the significant rise in Irish immigration in the 1830s and 1840s. Some Irish already lived in Chicago when it was incorporated as a city in 1837. In the next few years Irish numbers grew rapidly, particularly after the arrival of refugees from the Great Famine. By 1850 Irish immigrants accounted for about one-fifth of the city's population. Many of the city’s politicians are descendants of this group, including previous mayor Richard M. Daley. The Irish were able to assert themselves in politics due to their large population but also the fact that they knew English and that – thanks to the geographic position of Ireland on the periphery of Europe – they did not have ancestral ethnic rivalries. As the old saying went, "A Lithuanian won’t vote for a Pole, and a Pole won’t vote for a Lithuanian. A German won’t vote for either of them. But all three will vote for a turkey. – an Irishman." Only the WASPs hated the Irish, and the WASPs all lived on the North Shore." The Irish gained entry to Chicago's Fire and Police Departments and have kept family traditions of participation in these units. The Irish laid the foundations for many of the city's Roman Catholic churches, schools and hospitals. The Irish are still very active in the city's politics.
Germans have constituted a major portion of ethnic whites in Chicago since the beginning of the city's history. When the Great Plains opened up for settlement in the 1830s and 1840s, many German immigrants stopped in Chicago to earn additional money before moving West to claim a homestead. Those with skills in demand could—and often did—stay. From 1850, when Germans constituted one-sixth of Chicago's population, until the turn of the 20th century, people of German descent constituted the largest ethnic group in the city, followed by Irish, Poles, and Swedes. In 1900, 470,000 Chicagoans—one out of every four residents—had either been born in Germany or had a parent born there. Although their numbers dropped because of reduced emigration from Germany and because World War I had made it unpopular to acknowledge one's German heritage, 22 percent of Chicago's population still did so in 1920. One of the most distinct of these German groups were the Volga Germans, or ethnic Germans having lived along the Volga River in Russia. They largely clustered in Jefferson Park on the city's Northwest Side, coming to the area mostly between the years 1907–1920. By 1930 450 families of Volga German heritage were living in the Jefferson Park area, most of whom originated from Wiesenseite. Chicago also hosts the headquarters of the largest Lutheran body in the United States, the Evangelical Lutheran Church in America.
Poles in Chicago constituted the largest ethnically Polish population outside Warsaw before 1918 when Poland reemerged as an independent state. The city is one of the most important Polish diaspora centers in the 21st century, and every Labor Day weekend the city celebrates at the Taste of Polonia Festival in Jefferson Park. The Southwest Side is home to the largest concentration of Górals (Carpathian highlanders) outside Europe; it is the location of the Polish Highlanders Alliance of North America.
The city has one of the largest Assyrian Diaspora populations, with the Assyrian community of Metropolitan Chicago numbering as many as 80,000. Additionally, The Patriarchal see of the Assyrian Church of the East was based in Chicago for several decades up until 2015 when it was moved to Erbil.
Polish people have been prevalent from the city's early history, but the largest waves of immigration were in the late 19th and early 20th centuries; they have been influential in the economic and social development of Chicago. This is one of the most important centers of Polonia. The Taste of Polonia Festival in Jefferson Park celebrates Polish culture annually on Labor Day weekend. The Southwest Side is home to the largest concentration of Gorals (Carpathian highlanders) outside Europe. The southwest side is also the location of the Polish Highlanders Alliance of North America. Many Polish churches are found in Chicago, built in the Polish Cathedral style of architecture. Some can be seen from the Kennedy Expressway, other roadways, and public transportation routes, as well as from the neighborhood streets.
Chicago has one of the largest concentrations of Italian Americans in the US, with more than 500,000 living in the metropolitan area. Chicago has the third largest Italian American population in the United States, behind only New York City and Philadelphia. Chicago's Italian community has historically been based along the Taylor Street and Grand Avenue corridors on the West Side of the city. There are also significant Italian populations scattered throughout the city and surrounding suburbs.
The first Bosnians settled in Chicago in the late 19th and early 20th centuries, joining other immigrants seeking better opportunities and better lives. As the former Yugoslavia continued to find its identity as a nation over the last century, the people of Bosnia and Herzegovina sought stability and new beginnings in the city of Chicago many intending to return to their homeland. Bosnian Muslims were early leaders in the establishment of Chicago’s Muslim community. In 1906, they established Dzemijetul Hajrije (The Benevolent Society) of Illinois to preserve the community’s religious and national traditions as well as to provide mutual assistance for funerals and illness. The organization established chapters in Gary, Indiana, in 1913, and Butte, Montana, in 1916, and is the oldest existing Muslim organization in the United States. The Bosnians were the first Muslims in the United States to incorporate an Islamic Association in 1906 in Chicago, Illinois. There are approximately 70,000 Bosnian Americans in Chicago. 40,000 of them came as refugees during the 1990s and early 2000s.
Other prevalent European ethnic groups include the Czechs, and Ukrainians. At the turn of the 20th century, Chicago was the third-largest Czech city in the world, after Prague and Vienna. There are approximately 14,000 Ukrainians living within the Chicago city limits. Chicago has a small community of Swedish Americans. Swedish Americans make up 0.9% of Chicago's population, and they number at 23,990. After the Great Chicago Fire, many Swedish carpenters helped to rebuild the city, which led to the saying "the Swedes built Chicago." Swedish influence is evident in Andersonville on the far north side.
The city has a large population of Bulgarians, Lithuanians, Croats, Jews, Greeks and Serbs. Chicago has a sizeable Romanian American community, As of 2018[update], the Lithuanian population is over 100,000 and was formerly over 300,000; the world's oldest continuously published Lithuanian-language newspaper Draugus is based in Chicago. The city is the seat of the Evangelical Covenant Church and the Evangelical Lutheran Church in America. 
Chicago has the third-largest South Asian population in the United States, especially many Indians, Bangladeshi and Pakistanis. The Devon Avenue corridor on the far north side is one of the largest South Asian neighborhoods/markets in North America. As of the 2010 Census, Chicago has the third-largest Puerto Rican population in the continental United States, after New York City and Philadelphia, and the fourth largest Mexican population in the United States after Los Angeles, San Antonio, and Houston. There are about 185,000 Arabs in Cook County with another 75,000 in the five surrounding counties. Chicago is the center of the Palestinian and Jordanian immigrant communities in the United States, and additionally has a large Assyrian population.
The city saw an increase of 20,606 people from July 1, 2007 to July 1, 2008, according to census data. This marked the second consecutive year of population increase, while still not yet returning to the official Census 2000 population level. As of the 2007 US Census American Community Survey the largest European ancestries were:
|2000 Census Data||Chicago||Illinois||US|
|Population, percent change, 1990 to 2000||+4.0%||+8.6%||+13.1%|
|Median household income (1999)||$38,625||$46,590||$41,994|
|Per capita income (1999)||$20,175||$23,104||$21,587|
|Bachelor's degree or higher||25.5%||26.1%||24.4%|
|Hispanic/Latino origin (of any race)||28.9%||12.3%||12.5%|
There are more than 1,061,928 households, of which 28.9% have children under the age of 18 living with them, 35.1% were married couples living together, 18.9% had a female householder with no husband present, and 40.4% were non-families. Of all households, 32.6% are made up of individuals and 8.7% had someone living alone who was 65 years of age or older. The average household size was 2.67 and the average family size was 3.50.
Of the city population, 26.2% were under the age of 18, 11.2% were from 18 to 24, 33.4% are from 25 to 44, 18.9% from 45 to 64, and 10.3% 65 years of age or older. The median age was 32 years. For every 100 females there were 94.2 males. For every 100 females age 18 and over, there were 91.1 males.
The median income for a household in the city was $38,625, and the median income for a family was $42,724. Males had a median income of $35,907 versus $30,536 for females. The per capita income for the city was $20,175. Below the poverty line were 19.6% of the population and 16.6% of the families. Of the total population, 28.1% of those under the age of 18 and 15.5% of those 75 and older were living below the poverty line.
Population estimates in 2008 put the number of people in the city proper at 2,853,114, while suburban populations continue to grow, with estimates at 9,785,747 for the combined city and suburbs.
Chicago has the third largest LGBTQ+ population in the United States. In 2015, roughly 4% of the population identified as LGBTQ+.  Since the legalization of same-sex marriage in the State of Illinois in 2013, nearly 10,000 same-sex couples have wed in Cook County alone.
Christianity is predominant among the city's population who worship (71%). The Chicago metropolitan area also includes adherents of Judaism, Islam, Buddhism, Hinduism, Jainism, Sikhism, and the Bahá'í, among others.