The 20-year-old defender, who netted three goals in fifteen first-team appearances for the Saddlers last season, became a crucial member of the first-team during his six-month stay at Banks's Stadium and was a player Walsall manager Dean Keates had made no secret of his desire to sign.
Jack, who spent the first part of last season on-loan with League Two side Forest Green Rovers, was due to become a free agent at the end of the month and is keen to make the most of his opportunity to secure game time for the recently-relegated Baggies.
Speaking via West Brom's official website - WBA.co.uk - Jack said; "I'm really excited that it's all sorted and I can look forward to next season. I've always said that I want to be at this club. I've been here since I was seven and this is where I want to make it. Hopefully this is the time for me to break into the first-team and push into the starting eleven."