The Blues striker was allowed to begin talks with Paul Jewells side on Tuesday and has now completed a move to the JJB Stadium.
"The move came out of the blue a little bit but once it was obvious that Birmingham were willing to let me go it was an easy decision to come here," he told Wigan's official website.
"Leaving Birmingham is hard because I love the place, the fans and the people there but I believe I'm joining a club that is going places just like I did when I joined Birmingham three years ago.
"I'm good friends with Nicky Eaden and I know Andy Liddell and Neil Roberts too, so I always look out for Wigan's results anyway. They've had a great start to the season and hopefully I can come in and provide some more goals to keep us near the top of the division.
"From my point of view I could easily have sat on the last two years of my contract at Birmingham but that's not my style - I just want to play football.
"Coming here should give me the chance of first-team football and I can't wait to get started in training on Monday in preparation for my first game at Wimbledon."