The 1990 Illinois High School Association (IHSA) State Championship Tournament was held at Mahomet-Seymour High School. It was the fourth State Championship Tournament in Illinois, sponsored by the IHSA.
The state was divided into 16 areas, termed in Illinois as Sectionals. Each sectional hosted a tournament, the winner of which advanced to the State Tournament. The determination of these geographic boundaries was arbitrarily decided by the IHSA Office.
Each round was comprised of 30 toss-ups and accompanying bonus questions. The sectional tournaments and State Championship Tournament were all single elimination. Teams were randomly assigned to positions in the bracket without regard for record or performance.
There was no third place match. The top two teams earned trophies and medals for team members, coaches, principals, and activities directors. The trophies were of the design smaller than that of the athletic trophies given out by the IHSA.
- This was the fourth and final time the State Championship Tournament would be held at Mahomet-Seymour.
- IMSA did not play any matches this season prior to entering their sectional tournament. They won five rounds in the sectional, four at the State Championship Tournament, and became the first (and to date only) undefeated State Champion, with a record of 9-0.
- This was the second state title in a row for IMSA, joining Quincy with back-to-back state titles. It was the second of what would be eight titles by 2001.
- New Trier, Bloomington, Carbondale, and Homewood-Flossmoor all made their debuts at the State Championships this year.
- Joliet Catholic's second place finish was the highest finish of any Catholic school in the soon-to-end "Open Era", and among future Class AA teams, as of 2008.
- Carlinville's 209-208 win over St. Teresa marked the first one-point victory in a State Championship Tournament match. It would not happen again until 2003.
- Until 2011, this would be Lyons' only legitimate appearance at the State Championship Tournament (see Screwup on Western).