Gabriel Zakuani has signed a two-year contract at Peterborough, just five months after leaving the club.
The DR Congo international defender, 28, left the League One side in January to join Greek top-flight club Kalloni.
"I missed the club and this feels like a homecoming," Zakuani, who played 202 times during his first spell with Posh, told the club website.
"I feel like there is unfinished business here. I want to get this club back to the Championship."
The former Fulham and Stoke centre-back continued: "I really enjoyed my time in Greece, but I really missed the club and it hurt me when we were beaten in the play-offs. I can't wait to get started again."
Manager Darren Ferguson added: "Gabi expressed a real interest in coming back to the club and I am pleased to have him back on board.
"I think when he left, it was right for both parties at that particular moment. He has gone away and got experience of playing in a different country and done very well."