The Toffees have endured a torrid start to the season both domestically and in Europe, with yesterday's defeat at Goodison Park leaving them in the relegation zone despite heavy investment during the summer.
Everton spent almost £150m to bring in the likes of Wayne Rooney, Michael Keane, Gylfi Sigurdsson, Jordan Pickford, Davy Klaassen and Sandro Ramirez as they looked to breach the gap to the top six in the Premier League.
However, Koeman leaves having picked up just two Premier League victories all season, while the Merseysiders have also accrued just one point from their three Europa League group games so far this term.
"Everton Football Club can confirm that Ronald Koeman has left the club," read a statement on Twitter.
"Chairman Bill Kenwright, the board of directors and major shareholders Farhad Moshiri would all like to express their gratitude to Ronald for the service he has given to the club over the past 16 months and for guiding the club to seventh place in last season's Premier League campaign."
Koeman took charge at Goodison Park in June 2016 and went on to win 24 of his 58 games in charge, drawing 14 and losing 20.
Everton, who are winless in their last five matches across all competitions including four at home, face Chelsea in the EFL Cup fourth round on Wednesday night.