The 33-year-old former football player is leaving his role on the coaching staff at Tottenham Hotspur and to take his first full-time managerial position at the Championship side.
West Brom chairman Shilen Patel said: "We are delighted to welcome Ryan to The Hawthorns as our new Head Coach, along with Nigel [Gibbs] and Sam [Pooley].
"We have undertaken a thoughtful and expansive search and Ryan always stood out to us among an incredibly strong group of candidates.
"We are excited to have someone whose skillset means he has the aptitude to help us further establish elite standards in both preparation and player development for the first team and across all our team programmes.
"His experiences as both a first-team player and coach under some of the game’s most respected minds, along with his time leading academy sides, will be instrumental as we shape a clear philosophy and unified identity at all levels at the Albion."
The Baggies' sporting director Andrew Nestor thinks Mason's experience of playing football at the "highest level" as well as training emerging talent at the Spurs' academy will work to the West Brom players' advantage.
He said: "Ryan has had a unique and impressive football journey having played up to the highest level in the game, and developed his current game model under leading figures such as Mauricio Pochettino and FA Technical Director John McDermott.
"Following his retirement, he has honed his coaching abilities and his game tactics, working in recent years as part of Spurs’ academy with young and emerging talent, as well as with some of the best managers and players in world football at first team level.
"Ryan brings a style that will complement and continue to evolve our club’s game model. His approach to match tactics, individual player and overall squad development informed by data and analytics makes him an ideal fit, and he joins us at an exciting time as we establish a new foundation for the future success of the Albion.
"The appointments of Ryan, Nigel, and Sam further strengthen the work that has been undertaken in advancing our sporting structures. This began last summer with the reinforcement of our scouting and operations set-ups, and has since continued through to performance, sports science and analytics, with the principal aim of creating a cohesive unit to deliver sustainable competitiveness for the club."
