The 36-year-old joined the Seagulls on a free transfer from Liverpool in the summer of 2020 and has since made a total of 103 appearances across all competitions, chipping in with four goals and four assists.
Lallana has been a key figure in the Brighton dressing room since the arrival of head coach Roberto De Zerbi, although the midfielder has had to settle for a reduced role in the first team this season.
The 34-cap England international has played 29 times for Brighton across all competitions so far this season, including 12 starts and 12 substitute outings in the Premier League, while he also made a few outings across the Europa League, FA Cup and EFL Cup.
Lallana is yet to decide on his next move having now entered the twilight years of his career, but he plans to end the week on a high with Brighton in Premier League games against Chelsea and Manchester United at the Amex Stadium.
Lallana explains decision to leave Brighton
Speaking to Brighton's official website, Lallana said: "It was a tough decision, but it just feels like the right time, not just for myself but for the club as well. I've been in talks with Roberto for the past couple of months and he expressed his wish for me to stay and keep playing.
"However, after taking a little time to think about where the football club is at, and having been away from my family for the last four years, I just feel it's the right time.
"I've been speaking with my family throughout the year and the biggest influence on my decision was my kids, who need me around a bit more at the age they're at. I feel it's time in my life and in my career to put them first.
"Last week I told Roberto, the club and my teammates, who are extremely important to me. Now I just want to enjoy my last week, get a couple of really positive results, and finish the season on a high."
"I would like to say thank you for all your support these past four years," Lallana added. "We have experienced some unbelievable times. My message to our fans is keep supporting the lads, through the tough moments.
"The club is in an unbelievable position. Everything that's come out of this season: the financial numbers, European football, hopefully finishing in the top ten again, these are massive steps.
"It's important the fans remain patient and realistic in what we're doing and what we've done. The club's in safe hands with (owner and chairman) Tony Bloom, so trust the process and keep supporting the boys every week."
De Zerbi added: "We are losing a great man and great player. It has been his decision and we have to respect that. He has been a key player for me in my time here. At the beginning, Adam Lallana, Adam Webster, Pascal Gross, they followed me from the first day, and he will be a big loss for us.
"He is very smart, he loves football, he loves playing football. He understands the game as a player and as a coach, but I think he could play another season in the Premier League for sure."
Interestingly, Lallana only goal contribution for Brighton this season came against Wednesday's opponent Chelsea in the reverse fixture at Stamford Bridge where he captained the Seagulls and registered an assist in a 3-2 defeat.