Manuel Pellegrini has been sacked by West Ham with the club hovering just above the Premier League relegation zone.
A 2-1 defeat at home to high-flying Leicester City on Saturday proved to be the final straw.
West Ham are just a point above the bottom three and co-chairman David Sullivan said the board had to act.
"It is with great disappointment that we've had to make this decision. Manuel is a gentleman and it has been a real pleasure to work with someone of his calibre," Sullivan said.
"However, it has become clear that that a change is required to get the club back on track in line with our ambitions this season.
"We felt it was necessary to act now in order to give the new manager as much time as possible to try and achieve that goal."
Pellegrini arrived at the London Stadium in May 2018 on a three-year deal and led the club to a top-10 finish in his maiden campaign.
However, the Chilean – who guided Manchester City to Premier League glory in the 2013-14 campaign – has seen West Ham lose 10 of their 19 top-flight matches this term despite forking out big sums on the likes of Sebastien Haller and Pablo Fornals in the close season.
Former boss David Moyes is the early bookmakers' favourite to replace Pellegrini, while Bournemouth manager Eddie Howe and Rafael Benitez have previously been linked with the role.
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