The 34-year-old former Newcastle, Aston Villa and Fulham centre-half becomes new Seagulls boss Sami Hyypia's first signing.
Hyypia told the club website: "We wanted to bring another experienced defender, and Aaron fits the bill - having played at Premier League, Champions League and international level.
"I have played against Aaron a few times during my time at Liverpool and also for Finland, and I know his qualities and what he will give us.
"He has a very good footballing pedigree, is an intelligent player, and he has a great mentality and good approach to the game.
"He has that experience I wanted to bring into the squad, which alongside our other senior players, will help our younger players continue to develop and progress in the first-team squad.
"Overall, we still have some work to do in terms of recruitment, and we are all working hard to do that. This is a good start and Aaron is a great addition."