Chris Brunt will leave West Brom at the end of the season after 13 years at The Hawthorns.
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 Baggie 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.
In total he has played 419 times for Albion, scoring 48 goals, and has also earned 65 caps for Northern Ireland.
Dowling added: "In one sense it was a difficult conversation to have with Chris because I knew we were saying goodbye to a genuine club legend.
"But it was also made easy by Chris's professionalism and acceptance of the situation. In this day and age, to have stayed 13 years and logged more than 400 games, most of them at the highest level in the country, is astonishing. But football always moves on, Chris accepted that. He is disappointed not to have played more this season but he has not given up yet on making a contribution over these last nine games and has committed to finishing off the campaign doing everything he can to help us achieve our aims."
Brunt has retired from international football in August, 2018.
He played in eight of the games that saw Northern Ireland reach Euro 2016 - their first appearance in a major finals for 30 years. Despite a knee injury ruling him out of the tournament, the former Sheffield Wednesday player travelled to France to support his team-mates.
The Belfast man scored three goals for his country after making his debut against Switzerland in 2004. Michael O'Neill, Northern Ireland boss at the time, stated: "I am disappointed Chris is retiring as I believe he still has much to offer the team, however I completely understand his decision to spend a bit more time with his young family and to concentrate on his club career. I want to thank him for everything he gave me on the pitch. He was a model professional and his contribution to the Northern Ireland team over the past 14 years will be remembered for a long time."
O'Neill is now in charge of Stoke City and he could make a move for Brunt.
• Scottish Premiership clubs' first-team squads will be allowed to return to training from 11 June, the Scottish FA has confirmed.
The suspension of football, which has been in place since 13 March, will be lifted next week for top-flight sides.
Other leagues wishing to resume must prove they can meet testing and other health measures. Protocols issued to clubs this week say training must initially be non-contact and in small groups. Clubs are planning for the Premiership season to start on 1 August.