The 21-year-old has signed a 18-month contract and joined the Baggies on a free transfer after having been released by Derby in December.
Albion boss Roberto Di Matteo told his club’s website: “Giles has had injury problems over the past couple of seasons but has undoubted ability.”
Barnes has been struggling with injuries in the recent year.
Di Matteo added: “We’ve seen encouraging signs to offer him this deal, which has been structured to suit all parties.
“There is no question that if Giles can get his career back on track, we’ll have a very talented player on our hands.”
