The Austrian, who Wigan manager Roberto Martinez has brought in to play at centre-back, told the club's official website: "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."
"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 first joined Wigan midway through 2005-06 season, the club's maiden campaign in the Premier League, from Norwegian side Brann and went on to become a fans' favourite.
Departing as a free agent at the end of the 2009-10 season, he was then snapped up by West Brom, where he saw out a two-year contract before signing for German outfit Hamburg last summer.
Now back with the Latics, and in line to feature in Saturday's home league clash with Southampton, he becomes Martinez's fourth recruit of the January transfer window following the capture of the versatile Roger Espinoza from Sporting Kansas City and the loan signings of Manchester United striker Angelo Henriquez and Atletico Madrid goalkeeper Joel Robles.