Mason, 33, has signed a three-year deal at the Hawthorns and is tasked with getting West Brom promoted back to the Premier League.
“This is a huge club with a fantastic infrastructure and an incredible fanbase and I am excited about what we can achieve together,” Mason said in a statement.
West Brom finished ninth in the Championship last season and had been without a manager since the sacking of Tony Mowbray in April.
After his playing career was ended early by a serious head injury, Mason returned to Spurs as a coach in 2018.
He stepped up as caretaker boss in 2021 and 2023 following the sackings of Jose Mourinho and Antonio Conte.
Mason was part of Ange Postecoglou’s coaching staff as Tottenham ended a 17-year wait to win a trophy by lifting the Europa League last month.
“Ryan has been an integral part of Tottenham Hotspur for a number of years, both as a player and a coach,” said Spurs chairman Daniel Levy.
“I’d like to take this opportunity to thank Ryan for all his dedication and hard work over the years, and we wish him every success for the future,” he added. (AFP)