Guidi replaces Ignazio Abate, who left the post at the end of last season. He arrives from AS Roma.
He told the club's website: “Milan is a strong emotion for me, I don’t hide the fact that I’ve been a fan since I was little. Arriving here at Casa Milan and seeing so many photos, these colours that have accompanied me in my childhood and adolescence was something truly special for me.
“I would struggle to find the right words to describe what I’m feeling right now. It’s a source of great pride to be called by a prestigious club at a world level like Milan, and therefore I’m also aware of the great responsibilities that I’m taking on.
“There’s great happiness, now it’s a whirlwind of emotions but at the same time there’s a great desire to begin this magnificent journey.
“Roma for me was an extremely formative experience because it is a club that has given me so much in these two years, I had the opportunity to experience the daily work with mister Mourinho and then mister De Rossi.
“I was lucky enough to win important trophies such as the Coppa Italia and the Supercoppa. It’s a shame we weren’t able to close our journey by winning the Scudetto, but the thing I like most were the many debuts that we managed to make by bringing the boys into the first team, and I think that for us in the youth sector this is our mission and our greatest achievement.”