Scharner is returning to the DW Stadium having spent over five years with the Latics before leaving for West Brom in the summer of 2010.
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."
The 32-year-old can also play in midfield but is currently required in defence, where Martinez is missing the likes of Ivan Ramis and Antolin Alcaraz through injury and eager to strengthen.
The Spaniard, whose side are through to FA Cup fifth round and lie 18th in the Barclays Premier League, said: "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 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.
Martinez has indicated he would like to bring one more player in before this evening's 11pm deadline, although it appears it will not be Aiden McGeady, the Republic of Ireland winger currently with Russian side Spartak Moscow.
Regarding McGeady, Martinez, quoted by the BBC, said: "It's not going to happen.
"Aiden is a phenomenal footballer, but is a footballer that is playing his football in Russia. I don't think that's going to change."
Championship strugglers Barnsley have also confirmed they have given permission for the Latics to speak to their highly-rated 18-year-old defender John Stones.
The England Under-19 right-back has reportedly been tracked by a number of Premier League clubs and is thought to be valued at around £3million by the Tykes.
A statement on the South Yorkshire club's website, www.barnsleyfc.co.uk, read: "Barnsley Football Club have today given permission for Wigan Athletic to speak to John Stones.
"The young defender, a product of Barnsley Academy, has played for the first team 26 times this season and represented England at Under-19 level twice."