The England defender joined the Rossoneri on a permanent deal from Chelsea in July 2021 after a successful loan spell at San Siro.
Tomori put pen to paper on a four-year deal with Milan when he left Stamford Bridge a little more than 13 months ago.
The commanding centre-back played a big part as Milan ended an 11-year wait to win the Serie A title last season and he has been rewarded with a contract extension.
Chelsea academy graduate made 30 appearances for Stefano Pioli's side in the Italian top flight last season, helping the champion keep 13 clean sheets.
He won more tackles (44) than any other Milan defender, and had the most tackles won (44), blocks (23), interceptions (44) and clearances (77).
The 24-year-old's new deal was announced on the eve of Milan's first game of the Serie A season against Udinese at San Siro on Sunday (AEST).