Jones, who was named Preston’s player of the season, joins the Baggies on a Bosman after passing a medical.
The 24-year-old has signed a three-year contract with the Premier League side.
West Brom boss Roy Hodgson said: “I’m delighted to welcome Billy to the club.
“We’re really pleased we’ve been able to secure his signature in the face of stiff competition from other clubs for his services.
“It is great that he has chosen our club as the place to further his career and we are looking forward to working with him.
“We have been monitoring Billy’s progress for some time and, like the player himself, we believe this is the right moment for him to move up a level.
“He has been a very consistent performer in the Championship for a long period and this is his chance to show he can be as good a player in the Barclays Premier League.”
