The former Chelsea midfielder has signed an 18 month deal with the Rossoneri.
"I want to say thank-you, I'm happy to have joined the Milan family," he said.
"I have my best friend Sulley Muntari here and I've known him a long time. There is also Ricardo Kakà who I played with at Real Madrid last year.
"I'm looking forward to starting to play with the group and I can't wait to give everything to the group.
"I've known Seedorf for a long time, I asked him to a charity match in Ghana many times. Jose Mourinho said it would be a great experience and I would settle well in Italy.
"I told friends that before retiring that I wanted to play for this club and achieve my dream. It was easy to make a decision after speaking to Clarence.
"I had a lot of offers but Clarence made it easy choice for me.
"Milan always play to win trophies so I know I'm coming to a big club. I've chosen the number 15 shirt. I'm fit and ready to play.
"I just need to work hard to put himself up for selection for the coach. I'm looking forward to playing with all of the players.
"I'm excited to play in the Champions League here as I was not registered with Chelsea. I played a couple of times against Atletico Madrid but with the players we have here we can win the game against them."
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