David Moyes has been appointed West Ham manager for a second time after Manuel Pellegrini was sacked.
The Hammers are struggling in 17th in the Premier League, prompting Pellegrini's dismissal following Saturday's 2-1 home defeat to Leicester City.
Former Manchester City boss Pellegrini largely failed to get the best out of a talented group of players as West Ham slipped towards a relegation battle.
They have again turned to Moyes, who guided the London club to safety in the 2017-18 season. He has signed an 18-month contract, West Ham announced on their website.
The former Manchester United manager lifted West Ham to a 13th-placed finish in his previous spell in charge, but his short-term contract was not renewed and Pellegrini was appointed in May 2018.
Moyes, who has also managed Everton and Sunderland in the Premier League, has been out of management since leaving London Stadium.
He said of his return to West Ham: "It's fabulous to be back. It feels great to be home. I've missed being here because I really enjoyed it. I enjoyed being around the stadium and I loved being in this part of the world and I missed the club, so I can't wait to get started.
"I'm feeling very proud that I'm back here at West Ham. But I think more importantly I'll be looking to see what I can do and what I can make improvements to, how we can get some wins with the players and how we can get some quick wins on the field as well.
"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."
The next match for West Ham is the New Year's Day home fixture against Bournemouth, who sit just one point and one place better off than Moyes' team.
West Ham joint-chairman David Sullivan is confident the 56-year-old Scot, who was also recently linked with a return to Everton, has the credentials to turn around the club's on-pitch fortunes.
Sullivan said: "David proved in his short time with the club that he was capable of getting results and we believe that he will start moving the club in the right direction once again.
"We are delighted to welcome David back – 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."