The 27 year old became Owen Coyle's ninth signing of the summer and is likely to go straight into the squad for Saturday's league opener with Barnsley.
The former Luton Town and West Bromwich Albion defender has joined on a three year contract from Norwich City after falling out of favour with Canaries boss Chris Hughton.
New Latics boss Owen Coyle was clearly pleased with the piece of business as he told the gathered press: "As manager, to see the desire Leon has to play for Wigan Athletic is tremendous and we're all delighted to have enticed a player of his calibre to our club.
"He has shown his qualities in the Premier League on numerous occasions and has the experience of being successful in a division as tough as the Championship, winning promotion on three separate occasions.
"That kind of experience for someone who is still only 27 years old is fantastic and I believe that whilst he has much maturity in his play he also has huge potential to become an even better player here at Wigan Athletic."
Barnett is a defender full of pace and power who can play in a number of positions including midfield and full back, although his strongest position is at centre back.
His addition is almost a like-for-like swap with Maynor Figueroa in terms of defensive style and quality.