The 36-year-old, who has also played for Aston Villa and Manchester City, moved for an undisclosed fee having turned down the chance to extend his four-year stay at Everton.
He is five games from breaking ex-Manchester United winger Ryan Giggs' record 632 Premier League appearances.
"I want to help the team improve, work hard and hopefully add quality," Barry, who has 53 England caps, told the club's website (www.wba.co.uk).
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West Brom coach Tony Pulis believes Barry, who played 154 times for Everton having arrived from Manchester City on loan, is the ideal replacement for former captain Darren Fletcher who has joined fellow Midlands top-flight club Stoke City.
"There was a big void when we lost 'Fletch' and Gareth will fill that void,” said Pulis. "He’s a fantastic player and I think his attitude towards playing is really gauged by the fact that Everton had offered him a two-year contract to stay there.
Barry is West Brom's fourth signing of the transfer window following Jay Rodriguez, Ahmed Hegazi and China international Yuning Zhang, who has joined Werder Bremen on loan.
(Writing by Ken Ferris; Editing by Toby Davis)
