The former Southampton captain departed Anfield when the Premier League concluded on Sunday.
Lallana spent six years at Liverpool and won the Premier League, UEFA Champions League, UEFA Super Cup and FIFA Club World Cup at the club.
"Adam is a really exciting signing for us, someone I am sure the supporters will be really looking forward to seeing play as and when things are able to begin to return to normal," said Brighton head coach Graham Potter.
"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.
"I am really looking forward to working with him when we return for pre-season training in a few weeks."
"There is no doubting Adam's quality, and if you look at his footballing CV his ability is clear for everyone to see," Potter added.
"He has played at the very highest level, for both club and country, and achieved great things. He has a wealth of experience alongside his technical quality.
"The honours he won during his time at Liverpool, and the high regard in which he is held at Anfield, only further underline his ability and character."
Lallana will wear the number 14 jersey at his new club, who finished the 2019/2020 season in 15th place.
