The Serie A leaders have the option to make the move permanent for around £25million.
Thiago Silva’s summer arrival at Chelsea pushed Tomori further down the Stamford Bridge pecking order, with Kurt Zouma, Toni Rudiger and Andreas Christensen all also ahead of him.
The Canada-born defender opted to stay and fight for a first-team place before the start of the campaign, but has now grasped the chance of a big loan move to Italy.
Title-chasing Milan will offer Tomori a big chance to assert his top-level credentials.
Despite his temporary departure from west London, boss Frank Lampard has insisted that Tomori still has a “long-term future” with the Blues.
“Fikayo Tomori will spend the rest of this season on loan with Serie A leaders AC Milan,” Chelsea confirmed in a statement.
“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.”