The 25-year-old makes the move to the London Stadium on a loan deal until the end of the season and has gone straight into David Moyes' squad for the FA Cup fourth round tie tie at Wigan on Saturday.
Mario is a Portuguese international, notably forming part of the squad that won the 2016 European Championships, where he started sic of Portugal's seven games at the tournament.
The tricky playmaker will wear the number 18 shirt for the 'Hammers' and speaking to the club's official website, he says he can't wait to get started.
“I am really happy to be here. It’s a really amazing experience to join this amazing team and I am just glad to be here," he explained.
“It was a fast move. I have spoken with everyone around the team and, for me, it is exciting to play in the Premier League. I spoke with the coach and for me, I am really happy to be training for an amazing coach, also.”
Meanwhile, the midfielder, who is West Ham's first signing this month, revealed that compatriot Jose Fonte had been in touch to try to convince him to make the move to the Premier League.
“Jose explained to me a little more about the Club and also the song we have, I’m Forever Blowing Bubbles and that, of course, it was really important for me to join West Ham," he added.
“It’s a really historic Club with an amazing stadium and amazing players. I am just trying to know a little bit more about the Club every day and now I have some time to know more.”
Inter Milan paid Sporting Lisbon a reported £38.4m to seal the player's signature following a fine Euro 2016 but the player fell down the pecking order at the San Siro this season, featuring just 14 times in Serie A.
26 January 2018