News of the transfer came via the club's Twitter account, with the Ivory Coast international agreeing to move to the San Siro until 2019; the Rossoneri have an obligation to purchase him outright at the end of the loan period:
Kessie has cemented his reputation as one of the game's most exciting youngsters with some exceptional displays for Atalanta.
Here is a look at some of his best moments from what was a remarkable 2016-17:
The 20-year-old spent the 2015-16 season on loan at Cesena, and while he impressed during the temporary switch, there were few signs he was ready to take Serie A by storm.
But that's exactly what he did last term. Atalanta enjoyed a fine campaign, finishing in fourth position in the Serie A standings, and Kessie has been vital, netting six times in addition to registering three assists.
Football journalist Matteo Bonetti believes Kessie is exactly the type of footballer Milan need at this point in time:
Kessie is something of a throwback, as he's a player at the hub of the team who can do a bit of everything.
Indeed, Kessie plays with a vibrancy in the middle of the park, able to cut off passing lanes and hassle opponents in possession. It's on the ball where he's most effective, though, as the Ivorian possesses an excellent passing range, a turn of pace and feet that allow him to slalom past opponents.
As the aforementioned statistics show, Kessie has also showcased mature decision-making in the final third as well.
There are areas of his game that need to be improved, as was evident as the youngster struggled in the second half of the campaign, per OptaPaolo:
Manager Vincenzo Montella will be aware of that, but the Milan giants clearly recognise the remarkable potential Kessie has.
There are areas of his game that need to be refined, a process that'll come with age. But Montella will want to retain some of the raw features that made Kessie such a force for Atalanta. Should he strike a balance between the two in the years to come, Milan will have one of the division's best midfielders on their hands.