Roma acquired full ownership of the Italy international last week but have now sold the midfielder to their Serie A rivals
AC Milan have completed the signing of Andrea Bertolacci from Roma for an estimated €20 million.
The midfielder came through the youth team ranks at Trigoria but never made a senior appearance for the Giallorossi and had spent the last three seasons at Genoa as part of a co-ownership agreement.
However, Roma acquired 100 per cent of Bertolacci's playing rights last week, thus facilitating the sale of the Italy international to Milan.
Bertolacci underwent a medical with the Rossoneri on Monday before signing a four-year contract with the San Siro side later in the day.
"I have always said to [Milan] CEO Adriano Galliani that I wanted to come here and it's a great feeling to see it happen," he told the club's official website. "I'm ready to prove myself.
"It was always my first choice to come here. Galliani showed great enthusiasm to sign me. Now it will be up to us on the pitch to show we can win the Scudetto.
"I spoke on the phone with new coach Sinisa Mihajlovic and he has a great desire to bring back a Milan side of true greatness.
"I have excellent confidence after last season and at Milan I already have great friends, it's really good to be here."
Bertolacci netted six times in 34 appearances for Genoa during the 2014-15 campaign, with his fine form resulting in him breaking into Antonio Conte's Italy squad