The 36-year-old former England midfielder, who missed Everton's opening Premier League match of the new season against Stoke because of an ankle problem, has signed for the Baggies on an initial one-year deal.
"I want to come and help the team. That's always been the same for me wherever I have played," Barry said on West Brom's official website. "I want to help the team improve, work hard and hopefully add quality."
Everton boss Ronald Koeman was open to keeping Barry, but the club's summer spending has made competition for places fierce at Goodison Park.
The anticipated arrival of Gylfi Sigurdsson from Swansea, who looks set to complete the formalities of a £45million deal today, would see the former England international fall further down the pecking order.
Barry joined the Toffees from City on loan in 2013, helping them finish fifth before securing a permanent deal the following summer.
He said on www.evertonfc.com: "I have had four great years at Everton. The fans have been brilliant with me from the day I signed until the day I left. I am proud that the club will always be a big part of my career.
"There are too many people to thank, but I would like to mention a few of them - the chairman Bill Kenwright, (former manager) Roberto Martinez who brought me to Goodison, Ronald Koeman whom I have enjoyed working with, the staff at the club and some great team-mates - and of course the fans. Look forward to seeing you all again."