The defender, who has signed a four-year deal, spent the second half of the 2020-2021 season on loan with the Italian club and helped them secure Champions League football for the first time in seven years.
And Milan have exercised their option to make the move permanent when his contract with the Blues expires on June 30.
— AC Milan (@acmilan) June 17, 2021
The 23-year-old was impressive in his 22 appearances during his loan spell at the San Siro.
He helped the team keep seven clean sheets in Serie A, including five in their last five games, which saw them secure a second place finish.
The defender also made himself an instant hero by scoring in the 3-0 victory over Juventus in May.
Tomori has been with Chelsea since joining their U8’s in 2005 and he went on to make 27 first-team appearances as well as enjoying loan spells with Brighton, Hull City and Derby.
The centre back wrote a short message to the club on Twitter, which read: ‘Thank you for everything,’ followed by a blue heart emoji.
In a statement on their website, the club said: ‘Chelsea is grateful to Fikayo for his contributions at the club, from youth level to men’s first-team squad, and wishes him the very best in his future career.’
Tomori only managed one appearance for the Blues in the 2020-2021 campaign after being frozen out by Frank Lampard. The defender was a regular starter for his former boss at Derby and subsequently played 22 times when Lampard tookover at Stamford Bridge in the 2019-2020 season.
The defender spoke about his relationship with Lampard shortly after making his first move to Italy in January.
‘I will always be grateful for the opportunities he gave me,’ Tomori told The Telegraph.
‘Maybe because of our history he thought I would accept it (being frozen out), but from my perspective, with that history, it felt more personal and harder to understand.
‘I don’t know what happened, it wasn’t really explained to me. I was suddenly out of the squad and I don’t really know why.
‘I was trying to do extra and push myself more but, at that time, nothing seemed to work and nothing was explained to me.’
Rossoneri boss Stefano Pioli confirmed he wanted to secure the England youngster on a permanent deal in May after honorary vice-president and legendary centre back Franco Baresi had outlined their intentions to do so.
‘Tomori is imposing himself with talent and determination,’ Baresi said in March.
‘He has shown no fear playing in a new league. He can potentially become a great champion. I hope he lives up to expectations.’