The visit was his first public foreign foray since leaving the presidency.
Obama arrived yesterday, greeted by former Premier Matteo Renzi, a political ally who is mounting a comeback after resigning when his referendum on constitutional reforms failed.
Renzi welcomed Obama in a Facebook post, calling him "a friend and great leader ... Who still has much to say and give to world politics."
Obama made a few tourist stops in Italy's fashion and finance capital before attending a dinner hosted by an Italian think tank attended by friends and business leaders. He will give a speech on food security this afternoon.