The Baggies are believed to have paid a nominal fee for the midfielder, allowing Barry to fill the role left by Darren Fletcher in the middle of the park for Tony Pulis' team.
Barry, 36, had been described as a “great professional” by Everton boss Ronald Koeman and had been expected to play a role in the Dutchman’s squad this season despite the possibility of not starting every week.
Reflecting on his time with Everton, Barry told EvertonFC.com: "I've had four great years at Everton. 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.”
The midfielder signed off by stating: "Look forward to seeing you all again.”
He’ll have to wait a while before he returns to Goodison Park with Pulis’ side. The Blues and Baggies do not meet on Merseyside until January 20th.
But Barry will have his chance to play in front of the Everton away support once more as Koeman’s men travel to the Hawthorns a few weeks before, on Boxing Day.
A chance at history
Barry was on his way to becoming a squad player under Koeman despite appearing in 33 Premier League games in each of the past three seasons for the Toffees.
The 54-year-old manager had continually praised the former England international for his performances on and off the pitch and his professional attitude despite his role in the squad decreasing compared to the last few seasons.
The move to the Baggies will allow the 36-year-old to have a bigger role than he likely would have held at Goodison Park this season.
It also allows Barry to close in on Ryan Giggs’ Premier League appearances record of 632 games quicker than he would have with the Blues. Barry currently needs five more appearances to beat the Welshman’s record.
Will Everton fill the gap?
The Blues have already signed an abundance of central midfielders in recent windows and may not necessarily look to replace Barry with a like-for-like replacement straight away.
Muhamed Bešić and James McCarthy are both working their way back from injury and may just look to state a claim to a spot in Koeman’s squad.
Bešić had been seen as Barry’s long-term successor under Roberto Martinez but since his knee injury kept him sidelined for all of last season, it is unknown just how his future is viewed by Koeman and his staff.
The Bosnian international has filled in at centre-back during pre-season and could represent an option for Koeman if he is unable to secure a left-sided defender towards the end of the summer transfer window.
McCarthy, on the other hand, has been linked with a move away, most notably to Leicester City, and a sale could Everton back into the market for another centre midfielder. The Blues would be looking to at least recoup their initial £13 million outlay for the Republic of Ireland international.
Everton Director of Football Steve Walsh has been tracking Anderlecht midfielder Leander Dendoncker for a number of months but a log-jam in midfield has prevented the Blues from making a move for the Belgian international.