The 33-year-old centre-half has been one of Albion's best and most consistent performers this season, as reflected by a clean sweep of best-player awards at the end of the season dinner.
Boss Steve Clarke said: “I’m absolutely delighted the deal has been done.
“The fact Gareth has signed it tells you it is a good deal for him – and it is certainly a good deal for the club.
“He’s been a standout performer this season and deserves every reward he gets.
Gareth’s important to the dressing room. I’ve mentioned a number of times how strong the dressing room is and how we have a lot of good characters in the dressing room. Gareth is one of those.
“He’s important to the club and squad – both on and off the pitch.”