Following Sunday's Premier League loss against the Gunners, the club dropped to the relegation zone with just eight points from nine games played already. Everton also faces elimination from the Europa League tournament after picking up only one point in their opening three games.
The club on their Twitter page posted a statement confirming Koeman's sacking on Monday afternoon which read: "Everton Football Club can confirm that Ronald Koeman has left the Club.
"Chairman Bill Kenwright, the Board of Directors and Major Shareholder 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."
He bought Jordan Pickford, Michael Keane, Davy Klaassen and Gylfi Sigurdsson, but failed to impress the club with his new signing brought to the club.