The 19-year-old has penned a five-year deal with the Portuguese champions and says he is eager for success at Estadio da Luz
The 19-year-old, who came through the youth system at San Siro, completed a medical and signed a five-year deal with the Portuguese champions on Monday.
Cristante paid tribute to the Rossoneri for helping to guide him through his formative footballing years but is now relishing the prospect of competing for silverware at Estadio da Luz.
"I'm delighted to be here," he told BTV. "It was my choice. I want to thank president Luis Filipe Vieira, who wanted me, as well as the entirety of Milan - the president, Adriano Galliani, everyone who helped me grow.
"I know the history of Benfica. It is the best known and strongest team in Portugal, and I know the great history they have in Europe, having reached the final of the Europa League for the last two years.
"I want to win the championship and go as far as possible in Europe."
Cristante made three official appearances for Milan after his debut in 2011.