Though financial details were not released, French media reported the transfer fee could cost PSG up to €45 million ($55 million). The 27-year-old Silva had looked set to join PSG last month, but the move was blocked when he signed a new five-year contract with Milan.
"I'm very happy that Thiago Silva has signed," PSG coach Carlo Ancelotti said. "He is a fantastic player. Everyone knows that he is one of the best players in the world.
"With Silva and Alex, we have a good set of defenders and I will choose the best for the games. I'm not going to forget the other defenders who are working well."
Ancelotti had first signed Silva in December 2008, while at Milan. However, the defender never played under the Italian coach as he could not be officially registered until the following July, by which time Ancelotti had left to join Chelsea.
Silva went on to help Milan win the Serie A title in 2011.
PSG has been in talks with Milan over the transfer of both Silva and striker Zlatan Ibrahimovic this week for a reported combined fee of €65 million ($80 million).
