The midfielder, who joined from Sheffield Wednesday in 2007, will not have his contract renewed.
He has been limited to nine appearances this term and held talks with Albion’s technical director Luke Dowling about his future this week.
“It has been the honour of my career to have played for this great football club for the last 13 years but today I must begin saying goodbye,” said Brunt, who is aiming to win a third promotion with the Baggies this season.
“Before I go any further, I should make it clear that my entire focus over the next few weeks will be on doing everything in my power to get us back to the Premier League, to where this club and its fans belong.
“Though it may not come as a massive surprise to many, I thought it best we clarify the situation now, before we begin playing the final nine games of the season. We cannot afford to have any distractions from our goal.”
Brunt is one of only 16 players to have made more than 400 games for the Baggies and holds the record of Premier League appearances, 269, for the club.