The 23-year-old has joined the Rossoneri until the end of the season, Chelsea announced on Friday evening.
Milan are understood to have retained an option to buy Tomori for £25million in the summer, following the loan.
Title-chasing Milan will offer Tomori an opportunity to assert his top-level credentials, and he will wear the No 23 shirt at the club.
"The academy graduate will continue his development at the San Siro having been with Chelsea since under-eights level.
"He has made 27 appearances for the Blues so far, including four this season, most recently as a substitute in the FA Cup third round win over Morecambe earlier this month."