Sunderland and manager Martin O'Neill parted ways on Saturday following the team's 1-0 defeat to Manchester United at the Stadium of Light.
The result extended the Black Cats' winless streak to eight games in the Premiership and leaves Sunderland just one point above the relegation zone.
"Sunderland AFC has announced that it has parted company with manager Martin O'Neill this evening," a club statement said. "The club would like to place on record its thanks to Martin and wishes him well for the future. An announcement will be made in the coming days regarding a successor."
O'Neill spent a little over 15 months at the helm of Sunderland after joining the club in December 2011. But after adding both Steven Fletcher and Adam Johnson over the summer, while also bringing in Danny Graham and Alfred N'Diaye in January, results haven't followed.
"Of course, there is nobody that is happy with our current position and the way our season is going," said Sunderland chairman Ellis Short. "The reality is that we have a fight on our hands with eight games remaining. We know that we are capable of playing better than we have been doing and we know that we need to begin doing so now."