The 34-year-old, who joins from OFI Crete, has admitted that he is looking forward to the challenge ahead.
"I'm very happy to have joined such a club. It's a real honour to be part of Olympiacos, Greece's greatest team. I will do my best in order to contribute and help our squad accomplish its goals," Carroll told the club's official website.
"I'm really looking forward to the Europa League games and my own comeback to the European competition."
Carroll represented his country Northern Ireland 19 times before retiring in 2006.