With his backroom staff also leaving, Everton said under-18s head coach Leighton Baines and club captain Seamus Coleman would take charge of first-team affairs on an interim basis.
"Everton Football Club can confirm that Sean Dyche has been relieved of his duties as senior men's first team manager with immediate effect," the club said in a statement on Thursday.
"Ian Woan, Steve Stone, Mark Howard and Billy Mercer have also left the club. The process to appoint a new Manager is underway and an update will be provided in due course."
Everton were taken over by new owners, the Friedkin Group, in December and Dyche had said that he had their support.
But Everton are 16th in the standings after 19 games, a point above the relegation zone.
They have not won a game since beating Wolverhampton Wanderers in early December, a winless run of five matches. In all, they had won just one of their last 11 league matches under Dyche.
Dyche took charge at Everton in January 2023, having had a near 10-year stint as Burnley boss between October 2012 and April 2022.
The Toffees survived relegation at the end of the 2022-23 season by just two points.
Everton also overcame an eight-point penalty for breaches of the league's profitability and sustainability rules to finish 15th last season.
Jose Mourinho has emerged as an early contender to succeed Dyche. The Portuguese is currently with Turkish club Fenerbahce but has expressed interest in returning to the English game.
He also has experience of working with the Friedkin Group at Roma, although the Italian club sacked him just under a year ago.
David Moyes, who managed Everton between 2002 and 2013 before leaving to become Sir Alex Ferguson's successor at Manchester United, has also been linked with a return to Goodison.
With PA
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