The Baggies have signed the Austrian international on a two-year deal after he completed a medical at The Hawthorns on Monday.
Scharner's contract with Wigan ended at the end of last season, enabling West Brom to snap him up on a free transfer and Baggies boss Roberto di Matteo was delighted with the coup.
"I'm delighted to welcome Paul to the club," di Matteo told Albion's official website.
"He is exactly the type of player we've been looking to bring in because he is a proven Premier League player, having performed consistently well in the top flight for the past four-and-a-half years.
"Paul is a powerful, competitive player who is good on the ball, excellent in both boxes, can play in numerous positions and also chips in with goals.
"He is obviously short of match practice at the moment but when he's fully up to speed I'm sure his experience will prove invaluable as we strive to establish this club in the Premier League."