Northern Illinois is a region generally covering the northern third of the U.S. state of Illinois. The region is by far the most populous of Illinois, with nearly 9.7 million residents as of 2010.

Northern Illinois is dominated by the metropolitan areas of Chicago, the Quad Cities, and Rockford, which contain a majority (over 75%) of Illinois' population and economic activity, including numerous Fortune 500 companies and a heavy manufacturing, commercial, retail, service, and office based economy. Much of the economic activity of the region is centered in the Chicago Loop, the Illinois Technology and Research Corridor, and the Golden Corridor. However, rural sections of this region are highly productive agriculturally, and are part of the Corn Belt. The headquarters for John Deere farming equipment are located in Moline. Additional smaller cities in this area include Kankakee, LaSalle-Peru, Ottawa, Freeport, Dixon, and Sterling-Rock Falls, which still have predominantly manufacturing and agricultural economies. Northern Illinois is also one of the world's busiest freight railroad and truck traffic corridors.

Interstate 80 is sometimes referenced as the informal southern boundary of Northern Illinois, and is often used in weather reports as a reference point, as in "south of Interstate 80 will see sleet and rain, but north of Interstate 80 can expect mostly snow."

Interstate 88 (the Ronald Reagan Memorial Tollway) connects the region, east–west, stretching from the Quad Cities, eastward through Sterling-Rock Falls, Dixon, DeKalb, Aurora, Naperville, and into Chicago. Northern Illinois is also the only region of the state in which there are tollways, which are run by the Illinois State Toll Highway Authority, another trait separating this region from Central and Southern Illinois.

Northern Illinois University (NIU), in DeKalb, is located at the heart of Northern Illinois and is the state's second largest institute of higher education. According to the Regional History Center at NIU, their area of service to the northern portion of Illinois includes the 18 northernmost counties, excluding Cook, Grundy, Kankakee, Mercer and Rock Island Counties, which are covered by Eastern Illinois University and Western Illinois University, respectively, and University of Illinois at Chicago.

Several major colleges can be found in the Chicago area including Illinois' third largest state school, the University of Illinois at Chicago, as well as the University of Chicago and Northwestern University. Other notable schools include the Illinois Institute of Technology, Loyola University, DePaul University, Columbia College, Northeastern Illinois University, and Roosevelt University.

Several liberal arts schools such as Aurora University, Lewis University, North Central College, Elmhurst University, Wheaton College, Concordia University, and North Park University dot the Metropolitan Chicago landscape. Other institutions of higher education are found in Rockford, including Rockford University, Rock Valley College, Northern Illinois University-Rockford, University of Illinois College of Medicine-Rockford, a branch of Rasmussen College, and a branch of Judson University. Other colleges near the Quad Cities include Western Illinois University-Quad Cities and Augustana College.

These schools, along with several others, help to make Northern Illinois a vibrant research area. Such significant developments in science including the creation of the Atomic Bomb and the Fujita Scale were rooted in Northern Illinois institutions.