Tressel, 47, will replace Marcus Freeman, who left the Bearcats to join Notre Dame earlier this offseason.
Tressel has been a member of Michigan State's coaching staff since 2007. He's coached multiple positions and served as the team's defensive coordinator from 2015-19 under previous head coach Mark Dantonio. During the 2018 season, the Spartans boasted the eighth-ranked defense, allowing just 17.2 points per game.
Tressel was reportedly a candidate for the defensive coordinator position at Indiana. The Hoosiers ended up signing Georgia's Charlton Warren to fill the vacancy.
Tressel is the nephew of former Ohio State head coach Jim Tressel. He previously worked with Cincinnati head coach Luke Fickell in 2002 and 2003 as a member of Ohio State's coaching staff.
The Bearcats finished this past season as the eighth-ranked defensive unit, allowing 16.8 points per game. The lone blemish on Cincinnati's 9-1 record was a 24-21 loss to Georgia in the Peach Bowl.