The midfielder, who had fielded enquiries from Burnley, Newcastle, Leicester and Bournemouth amongst others in the top flight, passed his medical on Monday and will help shape the club’s tactical and psychological advancement under Graham Potter.
Brighton’s technical director Dan Ashworth led a seven-month pursuit of the player, who was credited by Jurgen Klopp as being one of the foundations of Liverpool’s success during his tenure.
Lallana was instructive to creating a culture of excellence in the dressing room and was active in nurturing the next generation of talent at Anfield, with Harvey Elliott, Neco Williams and Curtis Jones all underscoring how important he has been for their development.
“There is no doubting Adam’s quality, and if you look at his footballing CV his ability is clear for everyone to see,” Potter said.
“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.”
The 32-year-old, who said his goodbyes to his Liverpool teammates at Melwood on Friday, is enthused by the challenge of helping Potter’s side become more confident in possession. Lallana could also help them advance in terms of game management and generating belief that they can regularly beat better resourced teams.
He went through that psychological shift at Southampton under Mauricio Pochettino so is already well versed in it.
Lallana returns to the south coast, which is beneficial for his family. He crowned his six-year spell at Liverpool after joining in a £25m deal from St Mary’s in 2014 by becoming a Premier League, European and World Champion.
Lallana made 178 appearances for the Merseysiders, a figure that would have been higher but for injury troubles, directly contributing 43 goals.