The move sees Boumsong reunited with France team-mates Djibril Cisse, the former Sunderland forward, and Sidney Govou.
Boumsong: “I have come here to win titles.
“I don’t want to say that I am a leader, I only want to do the best I can for my new new team.
“I am really happy to come to Panathinaikos. I know that I will be playing alongside Djibril and Sidney, and I’m happy about that.”