The former Bologna midfielder joins Saints in a club record deal.
"I'm happy to be here at the Club," the 21-year-old told Saints Player.
"I came here because of the project I saw in this club - I believe in [Executive Chairman] Nicola Cortese's project.
"I know that just a few years ago the Club was in the third division, so they have had a great increase in success to be in the Premier League now.
"[Playing in England] was always my dream," he continued.
"I have been following the Premier League since before I was in Italy - that was a positive experience, but now I am delighted to be here in England and to play in the Barclays Premier League.
"Now, I am sure I can do very well for the Club and contribute to our success."