The 32-year-old has agreed a three-year contract to return to the south coast, having started his professional career at Southampton.
He will wear the No 14 shirt for Brighton next season.
“To have both his experience and quality out on the pitch will be a great addition for us and I know he will be an excellent role model for our younger players in the squad."
Lallana made 15 top-flight appearances this season while also playing in both matches as Jurgen Klopp’s side won the FIFA Club World Cup in December.
The midfielder has made a total of 196 Premier League appearances, scoring 30 goals and providing 28 assists.