Schalke have named Roberto di Matteo as new manager.
Di Matteo, who won the Champions League during a brief spell in charge of Chelsea in 2012, has been out of work since being dismissed by the Blues and he inherits an underperforming side.
Former Stuttgart boss Keller has been shown the door after just under two years in charge of the Royal Blues, with the club currently 11th in the Bundesliga table.
They are just three points above the bottom three and the same distance from the top six and were eliminated from the DFB-Pokal in the first round.
Saturday's 2-1 defeat at Hoffenheim proved to be the final straw for Keller, whose position was always under scrutiny from the day he replaced Huub Stevens.
