On 7 January 2018, Mirallas agreed to return to former club Olympiacos on loan until the end of the 2017–18 season. The club did not have an option to purchase Mirallas as part of the contract. On 16 April, Mirallas scored a stunning strike from the edge of the penalty area on the stroke of half-time in a 5–1 home win against Kerkyra. On 4 May, his loan spell was terminated by mutual consent due to Everton's high asking price to purchase Mirallas outright, as well as Mirallas' overall subpar performances with Olympiacos.
On 4 August 2018, Mirallas signed for Fiorentina on a season-long loan deal with the option to make the transfer permanent.
In May 2018, Mirallas was left out of Belgium's preliminary squad for the 2018 World Cup in Russia.