Diao moves to the Serie A outfit for a fee of €12m.
“I am very happy to be here and to become part of the Como 1907 family. I can’t wait to get started and give the fans lots of joy,” Diao said in an official statement on the club website.
“I have spoken to Mister Cesc Fabregas several times, we talked and he told me about the club’s project. I am very excited and I can’t wait to get on the pitch!”
Como coach Cesc Fabregas added: “Assane has a combination of speed, quality and an eye for goal. He is very intelligent off the ball and can find open spaces to free himself.
“His versatility will be a great asset to the team, I am looking forward to working with him. He is a player for the present and the future of the club. We continue to follow our strategy of having young, promising players in the team who can develop a lot.”