Mirallas will join the Greek outfit until the end of the 2017-18 season and he will hope to help them push for the Super League title.
The Belgium international has been limited to just five Premier League appearances this season, starting only two, and has not featured since November.
Ο ΟΛΥΜΠΙΑΚΟΣ συμφώνησε οριστικά με την Έβερτον για τον Κέβιν Μιραλάς! #olympiacos #KevinMirallas #Marinakis #leader #LeaderMarinakis #WeKeepOnDreaming #MiaGro8ia pic.twitter.com/kJCzu6MpaX — Olympiacos FC (@olympiacos_org) January 6, 2018
Mirallas is contracted to Everton until 2020, having signed a three-year deal last May, but leaves this month temporarily to return to his former side, who announced the deal on Twitter on Saturday.
"Olympiacos agreed with Everton for Kevin Mirallas," the tweet read.
The 30-year-old winger spent two years with the Greek giants, winning two Super League titles and the domestic cup, before making the switch to England with the Toffees in 2012.
He scored 29 goals in 151 Premier League appearances for the Goodison Park outfit, but has found himself left out of the squad since Sam Allardyce's arrival in November.
Mirallas had featured sparingly under Ronald Koeman and caretaker boss David Unsworth in the early stages as Everton struggled, but the current boss has had no reason to call him back as Everton have climbed from 16th to ninth in the league table since then.