Mirallas, 30, signed with the Toffees in 2012 from the Greek giants for a fee of £6m.
The forward has played 186 matches for the Merseyside club, scored a total of 38 goals and produced 34 assists.
However this season the Belgium international has struggled for game time and didn’t seem part of newly-appointed boss Sam Allardyce’s plans.
The Belgian has been wanting to return to Olympiakos since last summer but a move on transfer deadline day was denied.
He fuelled rumours of a Piraeus comeback in September when he appeared in Belgium’s clash with Greece in Athens wearing Olympiakos shin pads.
Mirallas joined the Greek club in 2010 on loan from Saint-Etienne and after a brilliant first season Evangelos Marinakis, who now owns Nottingham Forest, signed him on a permanent deal.
After a total of 71 appearances, 34 goals and 13 assists he left Greece for Everton.
He will now link up with Olympiakos’ new manager Oscar Garcia, who coached Brighton and Watford.
Mirallas, who won two league titles with the Greek side, is expected to receive a hero’s welcome from Olympiakos fans when his flight lands in Athens tonight.