Blue Island is a city in Cook County, Illinois, United States, located approximately 16 miles (26 km) south of Chicago's Loop. Blue Island is adjacent to the city of Chicago and shares its northern boundary with that city's Morgan Park neighborhood. The population was 22,558 at the 2020 United States Census.

Cook County is the most populous county in the U.S. state of Illinois and the second-most-populous county in the United States, after Los Angeles County, California. More than 40 percent of all residents of Illinois live within Cook County. As of 2020, the population was 5,275,541. The county seat is Chicago, the most populous city in Illinois and the third most populous city in the United States. The county is at the center of the Chicago metropolitan area.