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.
Former Everton and Manchester United boss David Moyes has been strongly linked with succeeding Bilic.
West Ham say Bilic’s coaching assistants Nikola Jurcevic, Edin Terzic, Julian Dicks and Miljenko Rak have also left the club with immediate effect.
“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,” added the statement.
Former Croatia defender Bilic, 49, was appointed in the summer of 2015 and guided West Ham to seventh place in his first season in charge.
But an 11th-placed finish last term, their first at London Stadium, has been followed by a poor start this season.
The Hammers have won just two Premier League matches in 2017-18 – and lost their first three league matches of the campaign.