The University of Illinois Urbana-Champaign (U of I, Illinois, or colloquially the University of Illinois or UIUC)is a public land-grant research university in Illinois in the twin cities of Champaign and Urbana. It is the flagship institution of the University of Illinois system and was founded in 1867. Enrolling over 56,000 undergraduate and graduate students, the University of Illinois is one of the largest public universities by enrollment in the nation.

The University of Illinois Urbana-Champaign is a member of the Association of American Universities and is classified among "R1: Doctoral Universities – Very high research activity", and has been listed as a "Public Ivy" in The Public Ivies: America's Flagship Public Universities (2001) by Howard and Matthew Greene.In fiscal year 2019, research expenditures at Illinois totaled $652 million.The campus library system possesses the second-largest university library in the United States by holdings after Harvard University.The university also hosts the National Center for Supercomputing Applications (NCSA) and is home to the fastest supercomputer on a university campus.

Illinois contains 16 schools and colleges and offers more than 150 undergraduate and over 100 graduate programs of study. The university holds 651 buildings on 6,370 acres (2,578 ha) and its annual operating budget in 2016 was over $2 billion.The University of Illinois Urbana-Champaign also operates a Research Park home to innovation centers for over 90 start-up companies and multinational corporations, including Abbott, AbbVie, Caterpillar, Capital One, Dow, State Farm, and Yahoo, among others.