Fickell, 37, had been the assistant head coach at Ohio State. He played for the Buckeyes from 1993-96 and has been a member of the coaching staff since 2002. He is the 23rd head coach in 121 years of Ohio State football.
"Ohio State is so much bigger than any team, player coach or situation," Fickell said Monday. "We all understand that change is inevitable. It's growth that's optional. And this is a time of growth for all of us."
Athletic Director Gene Smith said Fickell, who will have a base pay of $775,000 this season, is considered an interim coach. He will, however, be a candidate for the full-time position when a search is conducted following the season.
Ohio State is coming off a 12-1 season which gave Tressel a 106-22 record since he took over the program in 2001. The Buckeyes won seven Big Ten championships in that period.
But Tressel resigned May 30 amid allegations that he knew of potential NCAA rules violations, including players receiving improper benefits. Ohio State officials this summer go before the NCAA Committee on Infractions and Tressel could face severe sanctions if the committee determines he lied to investigators.