The former linebackers coach gets the promotion after Jeremy Pruitt left to take the same position at Georgia. Kelly will be in charge of the secondary, which Pruitt also coached.
Kelly joined the Florida State staff in 2013 after coaching defensive backs for the last three of his seven years at Georgia Tech. The 46-year-old has had previous stints as defensive coordinator at Jacksonville State, Henderson State, Nicholls State. He was the interim defensive coordinator for a half season at Georgia Tech in 2012.
Fisher said he has a linebackers coach and would make an announcement after completing requisite paperwork.
The Seminoles led the nation in scoring defense and interceptions en route to the national championship.