Latics confirmed they had completed the deal to bring Scharner back to the club he spent more than four years with before leaving in the summer of 2010.
The Austrian, who can play in midfield, but is set to be utilised by Wigan boss Roberto Martinez as a central defender, told Latics Player: "It feels as though I've come home.
"My first spell here lasted four-and-a-half years with many happy memories, so I'm delighted to get this opportunity to return.
"When I discovered that the manager wanted to bring me back, there was no question about it, it was an easy decision.
"I haven't been playing much in Germany with Hamburg and for me there is no better place in England to come to than Wigan.
"Since leaving I've had a few ups, and probably more downs to be honest but I'm still looking after myself fitness-wise and feel ready to give 100% to help us stay in the Premier League, my new challenge."
Wigan, currently 18th in the top flight, as well as being through to the fifth round of the FA Cup, have lost one centre-back in Ivan Ramis for the rest of the campaign due to a knee injury while another, Antolin Alcaraz, is yet to return to action from a groin problem.
Martinez has been keen to strengthen in the position and will now be able to select Scharner for Saturday's home league clash with Southampton.
The 32-year-old, a fans favourite during his first spell with the Latics, had the word 'THANX' dyed into his hair when he made his last appearance at the DW Stadium for the club and he is looking forward to greeting the Wigan supporters again.
"We've got a very important game this weekend against Southampton, and of course it's my first game back at the club which I'm going to work hard to be ready for," Scharner said.
"I'm looking forward to seeing the fans again and, if given the chance, I'll do my best on the pitch to try and get the win.
"I'm here to help the team and to help the club survive in the league, and with 14 games remaining I'm confident that we can do that.
"The last time I wore a Wigan Athletic shirt (at home) I put the word 'THANX' on the back of my head so I'm thinking maybe something like 'I'M BACK!' this time around!"
Scharner first joined Wigan midway through 2005/06, the club's maiden campaign in the Premier League, from Norwegian side Brann.
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 Hamburg last summer.
Martinez, who took charge at Wigan in 2009, is delighted to have the player at his disposal again and while admitting he is a unique character, the manager has also stressed that Scharner is a dedicated professional.
"You need those sorts of characters in football, although the way he is in the outside world probably distracts people from seeing what he really is day-to-day," Martinez said.
"He is a true professional with incredible attention to detail, so that his preparation is as good as it can be.
"He will bring those standards to the training ground and is someone who is going to be important for us."
Scharner is Wigan'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 says he would like to bring one more player in before Thursday's 11pm deadline, although he has indicated it will not be Aiden McGeady, the Republic of Ireland winger currently with Russian side Spartak Moscow.
Barnsley's highly-rated 18-year-old defender John Stones looks a more likely candidate, with the Championship outfit confirming they have given permission for Latics to speak to the youngster.
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.