The 32-year-old, who was a free agent after leaving the Baggies at the end of last season, has signed a two-year deal at the DW Stadium.
Wigan manager Owen Coyle has said that he is happy to bring Fortune to the Championship club.
"To bring a player of such calibre to the football club is a great piece of business," Coyle told Wigan's official website.
"He's shown his qualities at the very highest level, and when I spoke to [head coach] 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 excited to bring him in, the first of the strikers to arrive and I'm sure he'll bring a great deal to the team."
Fortune scored nine goals in 71 appearances for West Brom.