The 32-year-old’s Liverpool contract expired at the end of the season, with the midfielder departing a Premier League champion after six years at the club.
Lallana moved to Liverpool from Southampton in 2014, and spent the latter half of his spell on Merseyside on the fringes of the first team.
He moves to a club who secured their Premier League status before the final day on Sunday, with the Seagulls finishing the season 15th overall.
"I think we’re at the stage here where we can really push on in the Premier League," Lallana told Brighton's website.
"I have got a lot of experience playing in the Premier League and at a high level – I know what’s required and how we can improve."
Brighton head coach Graham Potter added: "There is no doubting Adam's quality, and if you look at his footballing CV his ability is clear for everyone to see.
"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.
"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.
"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."