In January this year, Mário signed for the Hammers on loan from the Seria A side for a €1.5 million loan fee, and an option for the club to buy the player for a sum in the region of €40 million He made his debut two days later, replacing Pablo Zabaleta at half-time in a 0–2 away loss against Wigan Athletic in the fourth round of the FA Cup.
João Mário scored his first goal for West Ham on 31 March 2018, helping the hosts defeat Southampton 3–0.He made 14 competitive appearances during his half-season spell, scoring twice, and was praised by supporters for his work rate.
Since his return to Italy Mario has been left frozen out of the Inter Milan team since comments he made to the media, he has often been left out of the squad completely and mostly been an unused substitute on the bench this season not managing one minute on the pitch in a competitive match. Unsurprisingly he was also left out of the Portugal squad in the recent International break.