The 32-year-old Austrian international, who played for the Latics from 2006-2010, will remain at the DW Stadium until the end of the season.
Scharner is Athletic's fourth signing of the January transfer window, following Roger Espinoza, Angelo Henriquez and Joel Robles.
Manager Roberto Martinez has been looking for a defender to replace the injured Ivan Ramis and the Wigan official website has indicated Scharner has been brought in to play at centre-back.
"His experience will be a huge asset going into the last third of the campaign and he knows our football club inside out the same way that we know Paul and his professionalism down to detail," read a statement on www.wiganlatics.co.uk.
"He is ready for the challenge and to help the team in an area where we have suffered a few injuries and we are slightly light in numbers especially at this moment of the season, facing the vital moments of two competitions."
Martinez is enthused by the return of the towering utility man, who also played in midfield for the Latics, adding: "There is no better centre-half for us to bring in than Paul Scharner.
"He knows the football club, we know him well, he has incredible experience and is a meticulous professional which means he is always in a great physical condition.
"He is desperate to come in and help the team and fight for his position. For us, it couldn't be a better option."
Scharner said: "It really is fantastic to be back. Wigan will always have a big place in my heart, so to be asked to come back and help the team now is something really special for me."
Only Emmerson Boyce (182) and Maynor Figueroa (169) have played more Premier League games for Wigan than Scharner, who has 145 appearances to his name.
Having signed from Brann in Norway in January 2005, Scharner left Wigan to join West Brom in 2010 before joining Hamburg two years later.