The 36-year-old midfielder underwent a successful medical at the Baggies' training ground this afternoon before putting pen to paper on a contract that takes him back to Birmingham where he spent a large chunk of his career with Aston Villa.
Barry was a regular England international in his prime and won the Premier League during his time at Manchester City before making the move to Goodison Park in 2013, first as a loan acquisition by Roberto Martinez and then a permanent signing.
He made over 130 appearances for the Toffees to pull within four of passing Ryan GIggs as the player to have played more games in the Premier League era than any other.
With Everton well-stocked for central midfielders and Tony Pulis having recently sanctioned the transfer of Darren Fletcher to Stoke City, Barry felt he had more chance of getting regular game time at The Hawthorns than at Goodison Park.
"I've had four great years at Everton," he told evertonfc.com. "The fans have been brilliant with me from the day I signed until the day I left. I'm proud that the Club will always be a big part of my career.
"There are too many people to thank but I'd like to mention a few of them - the Chairman, Bill Kenwright, Roberto Martinez who brought me to Goodison, Ronald Koeman whom I've enjoyed working with, the staff at the Club and some great teammates. And of course the fans.
"Look forward to seeing you all again."