Illinois Director of Athletics Mike Thomas, in a statement on the team Web site, confirmed the coaching change.
He said the decision was "difficult," especially considering the success Weber had early in his Illinois tenure.
"However, when making decisions of this magnitude, there are many things that need to be considered, including the direction of the program in terms of wins and losses, competitiveness in the Big Ten and fan support," Thomas said.
"Fighting Illini fans should expect our basketball team to be a factor in the Big Ten race and in the national conscience each season."
Illinois won its first 10 games this season and was ranked -- No. 25 -- for one week. The Illini posted wins against ranked teams Ohio State and Gonzaga and were 15-3 before the season began to unravel.
After a series of three close loses, Illinois defeated Michigan State on Jan. 31 but won just once more the rest of the season.
Iowa knocked Illinois from the Big Ten Tournament with a 64-61 victory Thursday. That left the Illini 17-15 this season.
Weber was 210-101 over nine seasons at Illinois. The Illini finished runner-up in the NCAA Tournament in his second season with the team. Illinois won two Big Ten titles -- Weber's first two seasons with the team -- and tied for second in the conference twice.
Before taking over the Illinois program in 2003-04, Weber coached five seasons at Southern Illinois. His overall coaching record is 313-155.
Thomas said the school would conduct a national search to replace Weber.