The 24-year-old joined the Rosonerri initially on loan from Chelsea in January 2021 before making the move permanent six months later.
Tomori signed a four-year contract when he made his move permanent last summer.
After helping martial Stefano Pioli's side's defence en route to their first Serie A title in eleven years last term, AC Milan have taken the decision to extend Tomori's deal by a further year.
A statement on the Italian champions' website read: "AC Milan is delighted to announce the extension of Fikayo Tomori's contract until 30 June 2027.
"Having joined Milan in January 2021, Fikayo proved his quality on the field immediately and earned the affection of the Rossoneri fans.
"Tomori and Milan will continue their journey together."
Tomori contributed 40 appearances across all competitions for AC Milan last season.
Tomori came through the Chelsea youth ranks and spent time on loan at Brighton, Hull City and Derby County before making a first team breakthrough at Stamford Bridge at the start of the 2019/2020 season under Frank Lampard.
Tomori has won three caps for the England national team and is hoping to be part of their traveling party to Qatar for this year's FIFA World Cup.
AC Milan begin their title defence against Udinese on Saturday.
