Andrew Crofts is leaving Albion to join Championship newboys Norwich City for an undisclosed fee.
The Seagulls will receive a significant sum for the 25-year-old midfielder, who joined the club on a free transfer from Gillingham last summer.
He will sign a three-year deal for Paul Lambert's side, who finished nine points clear at the top of League One.
Although reluctant to let his inspirational captain leave, Albion boss Gus Poyet acknowledges this is a terrific opportunity for the Wales international.
Poyet said, "We are really disappointed that Crofty is leaving us, because he was such an important player for us last season.
"When a team is playing well you inevitably attract attention from other clubs, and you can end up losing players.
"But that is football. This is a wonderful chance for Crofty to go and play in the Championship at a great stadium.
"Players only have short careers, and he feels he can't let this opportunity pass him by, so we have to respect that.
"We wish him well, and would like to thank him for all his efforts last season."
Albion chairman Tony Bloom added, "Naturally we are bitterly disappointed that Andrew is leaving us, because he had become an important part of the team.
"But we understand what a great opportunity this is for him, and we wish him well for the future."
Crofts made a total of 50 appearances for the Seagulls, scoring seven times.
Poyet appointed him as club captain after arriving as manager, and Crofts's influential performances in midfield eventually led him to be voted Player of the Season by Albion supporters.
