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.
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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.
Speaking after the chastening defeat at the hands of Jurgen Klopp's Liverpool on Saturday, Bilic admitted he did not know if he would be given time to turn things around.
"I don't know. It's hard for me to talk about that now," he said.
"Of course I believe in myself. I definitely don't feel a broken man. I'm very strong. I believe in my dedication and my previous work. I know my value.
"On the other hand the situation for West Ham is not good. The club is above every individual and that includes me.
"Are we playing well? No, we are not. It's me that has to take responsibility."
West Ham have just two wins from 11 Premier League games, leaving them languishing in 18th place in the table, above just bottom club Crystal Palace and Swansea.
The only bright spot in recent weeks was a notable victory against Tottenham at Wembley, when they came back from 2-0 down to win 3-2.
In his first season in charge, 2015-16, Bilic secured an impressive seventh-placed finish for West Ham, while last season they ended up in 11th spot.