Kessié has been a key figure in the midfield of Serie A outfit Atalanta this campaign, featuring in 28 league games with another appearance coming in the Coppa Italia.
Atalanta are currently six points ahead of AC Milan in the Serie A table, with Kessié's direct involvement in 10 league goals playing a huge part in their success.
Kessié is a powerful midfielder who loves a physical battle, while boasting an impressive technical ability and passing range. The youngster, who can also play as a centre-half, already has 15 international appearances for the Ivory Coast national team.
Milan fought off competition from across Europe to sign the midfielder, including strong interest from AS Roma.
Kessié will join a selection of young prospects at the San Siro as AC Milan aim to reclaim their spot amongst Europe's elite. Young stars Manuel Locatelli, Gianluigi Donnarumma and Alessio Romagnoli could all join Kessié on the field for Milan next season.