The 27-year-old becomes the Seagulls’ third signing of the summer on a free transfer after his short-term stay with Wigan came to an end in May.
Brighton manager Chris Hughton told the club’s official website: “Gaetan is a player that we were aware of before he came to England, while he was at Olympiacos in Greece.
“When he joined Wigan in February it provided us with the ideal opportunity to monitor his progress in England.
“That gave us a really good opportunity to watch him at close hand in the Championship, and we saw him a lot during that time and he was one of the outstanding players for them.
“Gaetan has played top-flight football in France and Greece, and has experience at international level – which has been key for us this summer.
“He is very athletic, he is a natural left-sided player, and it is important to have that balance in the squad.”
Bong started his career with Metz and also played for Tours and Valenciennes in France before moving to Greek giants Olympiacos in 2013. He joined Wigan on a free transfer in February 2015 and made 14 appearances for Latics.