The 20-year-old is a product of Roma’s academy and made his senior debut in December 2012, before recently impressing during his season-long loan spell at Sampdoria, for whom he made 30 league appearances and scored twice.
Romagnoli signed a new four-year contract with Roma in May 2014 but has now switched allegiances to Milan, who on Tuesday afternoon announced his arrival at the San Siro, where he will stay until 2020.
Vice-president Adriano Galliani welcomed the youngster to Milan, but warned he will have to work to force his way into the first team.
“Welcome Alessio, you have signed a contract with us until June 30, 2020 and that’s something we’re all very happy about,” he told the club’s official website.
“You’re still only a boy, you’re 20 years old, and you’ll need plenty of time before becoming a mainstay in our team, someone who can look after the ball like one of the greats from the Rossoneri school of defending.”