He leaves Albion at the age of 40 as his contract expires.
Milner made a record 658 appearances in the Premier League, the last of them in the Seagulls’ season-ending 3-0 home defeat by Manchester United.
He started his career with Leeds and went on to win three Prem titles, two with Manchester City and one with Liverpool.
His time at Albion was badly hindered by injury but he had a big impact around the club and off the field even when not playing.
He then stepped into a midfield role to help inspire a big turn around in form for the Seagulls as they climbed from the bottom half of the table and into a European place.
Milner posed on social media: "I've been fortunate enough to experience some unforgettable moments, from fighting for survival to winning trophies, playing in Europe, and representing my country at two European Championships and two World Cups.
"But more than anything, it's the people and friendships I've made throughout the game that I'll cherish forever."
Head coach Fabian Hurzeler told The Argus during the last season: “James Milner is someone who challenges everyone, who has the winning mentality, winning culture inside of him.
"That’s something we need here in this club because we have a clear vision and exactly players like this, who had like this career, who had like this success in their career, they are so important for the culture of the club.
“Not only because of the past, because generally by his behaviour, by his leading style, by his action he always shown, by his messages he always sends to the team during the week and also on matchday.
“So I’m really happy that he’s here and hopefully he will be here longer. But, in general, it’s great to work with a person like James in the building.”