Napoli sign Scottish star McTominay from boyhood club Man Utd
Owner Aurelio De Laurentiis confirmed the transfer on social media with a post which read "Welcome Scott!" alongside a photo of the 27-year-old signing a reported four-year contract worth three million euros a season.
Italian media report that Napoli have agreed to pay 30.5 million euros (£25.7m) for United youth team graduate McTominay, who eventually played 255 times for his boyhood club.
McTominay, who has 52 caps for his country, won the League Cup and FA Cup during his seven years with the United first team.
He is Napoli's sixth signing of the summer transfer window, which closes at midnight on Friday in Italy, after Romelu Lukaku, David Neres, Alessandro Buongiorno, Rafa Marin and Leonardo Spinazzola.
Napoli, who host Parma on Saturday, are trying to bounce back from a dreadful league title defence last term in which they went through three managers and finished 10th.
Antonio Conte took charge this summer and his new team currently sit mid-table in Serie A after a crushing defeat at Verona and convincing win over Bologna in their first two matches of the new season.