Sunderland have sacked manager Gus Poyet after a run of just one victory in 12 Premier League games.
The Black Cats are one point above the relegation zone after Saturday's 4-0 home defeat by Aston Villa.
"Sadly, we have not made the progress that any of us had hoped for this season," said chairman Ellis Short.
"We find ourselves battling, once again, at the wrong end of the table. We have therefore made the difficult decision that a change is needed."
A Sunderland statement added that an announcement about Poyet's successor would be made "in due course".
The Wearsiders face a trip to West Ham on Saturday before a derby against Newcastle on 5 April.
Poyet, 47, took training on Monday morning and then met chief executive Margaret Byrne and other board members to learn his fate.
Poyet had taken over from Paolo Di Canio in October 2013 with the club bottom of the Premier League.
|Gus Poyet's Sunderland record|
|8 Oct 2013||75||23||22||30||80||104||30.67|
But four victories and a draw from their final six league games helped keep them up - a run that included wins at Chelsea and Manchester United, plus a draw at Manchester City.
Uruguayan Poyet, who has managed Brighton, also led the Wearsiders to the 2014 Capital One Cup final, which they lost to City.
"I would like to thank Gus for his endeavours during his time at the club, in particular last season's 'great escape' and cup final appearance, which will live long in the memory of every Sunderland fan," said Short.
|Sunderland's season of woe|
|Seven of their games have ended 0-0 - no other team has recorded more than four.|
|Only Aston Villa (19) have scored fewer goals than Sunderland (23).|
|Sunderland have had the fewest number of shots (204).|
|The Wearsiders have the second-worst home record in Premier League.|
This term, Sunderland have just four wins from 29 league matches and were knocked out of the FA Cup by League One Bradford in February.
Following that 2-0 defeat by the Bantams, Poyet wrote an open letter to fans calling for unity to help their battle to stay in the Premier League.
Sunderland were 4-0 down to Villa before half-time on Saturday.
Some angry fans had to be restrained by security staff as they tried to get close to Poyet in the dug-out, while other supporters left the ground early.