West Ham sacked manager Slaven Bilic on Monday after a poor run of results that have left the club in the Premier League relegation zone.
“West Ham United can confirm that Slaven Bilic has today left his position with the club,” the Hammers said in a statement on their website.
The club hierarchy “believe a change is now necessary in order for the club to move forward positively and in line with their ambition”, the statement added.
The Croatian, who was appointed in June 2015 on a three-year contract, suffered a 4-1 loss to Liverpool on Saturday in his last game in charge.
Former Everton and Manchester United boss David Moyes is widely expected to replace Bilic at the London Stadium.
The West Ham statement added: “The club’s search for a new manager to take West Ham United forward is under way and an announcement regarding Bilic’s successor is expected to be made over the coming days.”
Bilic’s assistants Nikola Jurcevic, Edin Terzic, Julian Dicks and Miljenko Rak have also left the club with immediate effect.