The 32-year-old becomes Latics manager Owen Coyle's sixth signing of the summer so far, with Baggies boss Steve Clarke reluctant to lose him at the end of his contract
The 32-year-old, who was released by West Brom upon the conclusion of the 2012-13 campaign, has signed a two-year deal with the Championship club.
He becomes Owen Coyle's sixth signing of what has been a busy off-season at Wigan, following Scott Carson, Stephen Crainey, James Perch, Chris McCann and Thomas Rogne. to the DW Stadium.
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"He's shown his qualities at the very highest level and, when I spoke to Steve Clarke at West Brom, he told me how he didn't want to lose him, which goes to show how important he has been at his former club and proof that he has the capabilities to be a Premier League forward, never mind a Championship one.
"So I'm really exciting to bring him in, the first of the strikers to arrive, and I'm sure he'll bring a great deal to the team. He's powerful, can play anywhere across the front line and has pace to hurt defences.
"With the amount of games we have to play this season, it's important to have these kind of players at the club."
Fortune scored just two goals in 23 appearances for West Brom last term, finding himself behind Romelu Lukaku and Shane Long in the pecking order for large parts of the season.
The Frenchman also previously spent two campaigns with Celtic but left the Glasgow club after falling out of favour with Neil Lennon.