The 32-year-old former Liverpool man, who is currently with German side Wolfsburg, followed Manchester City full-back Wayne Bridge into the Stadium of Light.
O'Neill said: "Sotirios offers a real physical presence, coupled with a wealth of experience, which will be an undoubted benefit to us in the next few months. I'm delighted to welcome him to the club and I'm sure he is looking forward to the challenge of returning to the Premier League."
However, unlike Bridge, whose deal was completed before the noon cut-off point, the 6ft 4in Greece international will not be available for Wednesday night's Barclays Premier League clash with Norwich.
Kyrgiakos began his career in his home country with Panathinaikos and spent two of his initial seven years in Greek football on loan at Agios Nikolaos.
After winning his domestic double in 2004, he joined Scottish side Rangers and collected league and Scottish League Cup winners' medals in his first season.
Kyrgiakos joined German outfit Eintracht Frankfurt in 2006 and had a season back at home with AEK Athens before heading for Liverpool, where he made 30 league appearances, during the summer of 2009.
Source: PA
Source: PA