"I cant wait to get started," Lallana said. "It's a breath of fresh air coming here – having my medical, everything is new. It's a challenge I am looking forward to.
"The whole infrastructure and ambition around the club excites me. There is so much young talent and so much ability.
"The manager is highly regarded and it was a pretty easy choice for me in the end, especially knowing how much Brighton have reached out. It's nice to know that you're wanted – I think it's a perfect match."
He added: "Brighton are on an upward curve. I have played against them a lot, and remember seven or eight years ago when Southampton and Brighton were both in League One, so they have been progressing for years now.
"I think we're at the stage here where we can really push on in the Premier League. 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. If I can help just a little bit in that journey to improve Brighton as a whole, and as a squad, then I will be very happy."