The midfielder signed a short-term contract extension at Anfield in June with manager Jurgen Klopp wanting him to stay and "finish the job" of delivering the Premier League title.
Since arriving to Liverpool from Southampton in 2014, Lallana has won the Premier League, Champions League, UEFA Super Cup and Club World Cup.
"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 manager 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."
Lallana was the second departure from Anfield on Monday following Croatian defender Dejan Lovren's move to Zenit St Petersburg
Lallana has made 22 appearances in all competitions this season, but failed to feature in Liverpool's last eight league matches ahead of his anticipated departure.
Before Liverpool lifted the league trophy, Lallana told the club's website that he was sad about leaving at the end of the campaign.
"There's definitely a sadness obviously," Lallana said. "There are parts of me that are excited about my next challenge which I'm close to knowing what's happening so there's a mixture of emotions really at the moment.
"But what I keep bringing it back to is just making sure I enjoy every last day of my last week really because there's a lot happening.
"I think I've lost three finals here, they were all one after the other, so to be leaving having won four trophies is somewhat remarkable really, I still can't believe it.
"So, I think it's kind of a fitting time to leave and end my chapter here. Like you mentioned there's been plenty of highs but also plenty of lows as well that have been difficult.
"So, to leave with the trophies and with the amazing memories, with so many friends as well that I'd probably class as family, makes it special and fitting I think."
Brighton finished 15th in the Premier League, which concluded play for the 2019-20 season on Sunday. England's top flight returns to action for the 2020-21 campaign on Sept. 12.