Mario, who scored six goals in 33 league appearances for Sporting last season, played a key role in helping Portugal win Euro 2016.
"We're delighted to have Joao Mario at Inter. He's a top player and we'd been tracking him for a long time. He'll further reinforce what is already a very strong squad," the Italian club's president Erick Thohir said.
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(Reporting by Ian Rodricks in Bengaluru, editing by Ed Osmond)
