The move was made after a meeting between club president Martin Kind, sporting director Dirk Dufner and the members of the Hannover board.
"We didn't make the decision lightly," said Dufner. "However, we're convinced that something had to change." Hannover sit just four points above the relegation zone in 13th place at the halfway stage, having amassed just 18 points from their first 17 Bundesliga games this term. The team's away form has been particularly poor, with the 96ers losing all eight outings so far, conceding 20 and scoring just six.
"My personal thanks go to Mirko Slomka," said Kind. "He saved Hannover 96 from relegation and led the side into the UEFA Europa League twice. These achievements will always be associated with his name."
Slomka's contract was due to run until 30 June 2016. His backroom staff have also been released by the club.