"I'm looking forward to starting to play with the group and I can't wait to give everything to the group," he told reporters.
"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 are currently ninth in the Serie A table.