On 2 June 2017, Kessié joined fellow Serie A side AC Milan on a two-year loan deal with the obligation to buy. Originally, he chose 19 as his shirt number but soon was persuaded by the club’s management to leave it for Leonardo Bonucci, who transferred to Milan a few weeks later; as a result, Kessié changed it to 79. Following Bonucci’s departure in the next summer transfer window, he was offered his original shirt number back yet refused, citing that he does not want to force Milan fans to spend any extra money on his personalized shirts due to yet another number change
On 12 September 2021 against Lazio, Kessie won a penalty kick but eventually failed to score it, Milan won 2-0. On 28 September, in a UEFA Champions league match against Atletico, as Milan was leading by 1-0, Kessie was sent off in the 29th minute, his side eventually lost 1-2 On 31 October against Roma, he converted a penalty kick to score the second and seal a 2-1 victory for Milan. On 7 November, while playing against inter he fouled Calhanoglu in the penalty box, causing a penalty kick which the latter scored in 1-1 draw.On 4 December, Kessie scored the first goal against Salernitana in eventual 2-0 win.On 22 December, Kessie scored a brace against Empoli, he scored 14 Serie A goals in 2021; the last AC Milan midfielder to score so much in a single calendar year was Kaka in 2008 (15). Starting from Empoli’s match, Milan’s coach Pioli started to deploy Kessie as playmaker instead of his usual position in the center of midfield.
FC Barcelona showed interest in him and now verbal agreement have been reached, Frank Kessie has signed as new Barcelona player until June as free agent from Ac milan.
Kessie will receive €6.5m net salary plus add-ons. Medical have already been completed.