McTominay, 27, has spent his entire club career to date at Old Trafford, having first become involved with the United youth system aged five.
The midfielder, who made his Red Devils debut in 2017, racked up 255 appearances in total for the club and never even spent time away on loan.
After news of his move was announced, United penned an emotional tribute to McTominay on social media.
The Scottish midfielder had spent his entire career at Manchester United, playing 255 times
'The Red who lived his dream,' the post read. 'Your United family wishes you nothing but success in this new chapter @McTominay10.'
In Italy, McTominay is set to link up with former Chelsea and Tottenham boss Antonio Conte.
Conte has already signed former Premier League star Romelu Lukaku this summer, while Napoli are also chasing Brighton's Billy Gilmour ahead of tonight's deadline.
Selling McTominay should free up space and funds for United to complete their pursuit of PSG's Manuel Ugarte, who the club have agreed to sign in a deal worth £42m.
McTominay had also become a cult hero within large sections of the United fanbase.
He scored twice in stoppage time after coming off the bench to help Ten Hag's side beat Brentford 2-1 last season but his most memorable moment in the red shirt arguably came when his long-range effort sealed victory over Manchester City in March 2020.
The boyhood Red leaves Old Trafford having scored 29 goals across 255 matches for United.