Tommy Elphick is joining AFC Bournemouth for an undisclosed fee.
The 24-year-old central defender, who signed a one-year contract extension with Albion in March this year, has agreed a three-year deal with Cherries.
Brighton-born Elphick missed the whole of last season with an Achilles injury, but recovered in time to start pre-season with the rest of the squad.
Gus Poyet said, "Having given Tommy a one-year contract towards the end of last season, he has come back from a major injury in pre-season and proved himself.
"However, he is keen to play regular first-team football and with Gordon Greer, Lewis Dunk and Adam El-Abd all vying for those positions, we cannot guarantee him that.
"I understand he wants to play, and not sit in the stands and therefore I thank him for his service to the club and wish him the very best with Bournemouth in the future."
Chairman Tony Bloom added, "Tommy has been with the club since he was a schoolboy, and I am sure it was not an easy decision for him to leave.
"We have seen him rise up through the ranks, and he has been a magnificent servant to the club at youth and senior level, playing a significant part in the four years leading up to the club's promotion to the Championship in May 2011, just before his terrible injury.
"He is a shining example to youngsters off the pitch with the work he's undertaken as part of the club's schools project and Albion in the Community.
"On behalf of all Albion supporters I'd like to thank Tommy for his contribution to the club and wish him all the best for the future."
Elphick was handed his debut by Mark McGhee as a substitute at Reading in December 2005, while still a member of the youth team.
After winning his first professional contract at the end of that season, Elphick made his full debut against Doncaster in 2007, and soon became a first-team regular under boss Dean Wilkins. He was voted Player of the Season at the end of the 2007/08 campaign.
In total Elphick made 180 appearances for the club, scoring nine times - including the memorable first-half equaliser at Aston Villa in the FA Cup.