Adiyiah attracted the Serie A club's attention by helping Ghana win the Under-20 World Cup last month in Egypt, where he was the tournament's top scorer with eight goals.
"Having been the best player at the Under-20 World Cup is a great thing. I'm happy about his arrival and I think he'll do well," Leonardo told a news conference. "He's young. He's a buy for the future."
Adiyiah's contract will become valid when the transfer window opens in January.
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