David Moyes has been appointed as West Ham manager and will return for a second spell at the club.
The Scot, 55, has signed an 18-month deal and starts with the New Year's Day home game against Bournemouth.
He replaces Manuel Pellegrini, who was sacked after Saturday's 2-1 home defeat by a much-changed Leicester side.
"I feel like I've got unfinished business," said Moyes, who led the Hammers to Premier League safety in a seven-month spell from November 2017.
Former Everton and Manchester United boss Moyes, who took over from Slaven Bilic two years ago, left at the end of the season as the east London club decided to "go in a different direction" with former Manchester City manager Pellegrini.
But the veteran Chilean, who led the Hammers to a 10th-place finish last season, was sacked following their fourth straight home defeat on Saturday.
It left them one place and one point above the relegation zone.
"I'm feeling very proud that I'm back here at West Ham, but more importantly I'll be looking to see what I can do, what I can make improvements to and how we can get some wins with the players," Moyes said.
"I do believe that the squad of players I've got here is a better squad of players than when I took over before, so I am looking forward to working with them.
"I think there's got to be a short-term goal for us just now to get the wins to get us away from the wrong end of the league and what we're going to do over the next six months, then what we're going to do over the next period as well. The start is really focusing on the immediate games."
Stuart Pearce and Alan Irvine, who were Moyes' assistants in his previous stint, are again likely to join his coaching staff.
"David proved in his short time with the club he was capable of getting results and we believe he will start moving the club in the right direction once again," said Hammers joint-chairman David Sullivan.
"He knows the club well and he built strong relationships during that time which will be crucial for the work that needs to be done going forward."
'I believe in David Moyes'
Speaking on Match of the Day 2, former Arsenal defender Martin Keown said: "I think he's not a bad manager, he finished 13th with them a couple of seasons ago and he knows most of the players.
"You could say it is a backwards step but I believe in David Moyes and he'll come back this time maybe a better manager."