The 24-year old midfielder has penned a five-year contract which will keep him at St Mary's until 2018, joining Kelvin Davis, Rickie Lambert, Morgan Schneiderlin, Jason Puncheon and Luke Shaw in committing his long-term future to the Saints.
"I'm absolutely delighted," Lallana told the club's official website. "To commit my future to the club for the next five years is something I'm really happy about.
"I can't wait to work with the new management team and the lads to continue our progress in the Premier League. It was an easy decision for me with the direction the club is going in.
"I've been at the club since I was 12 and - being honest - I can only see my future lying with Southampton Football Club.
"If someone was to say that I'd end my career playing for Southampton in the Premier League then I'd snap your hand off for it.
"I am privileged and honoured to be given the opportunity to be here for the next five years. It's something I'm going to look to grab with both hands.
"I'm honoured to be playing for Southampton and captaining the club at the present moment, and I just hope the next five years don't go as quickly as the previous five!
"I can't wait to get going and to continue playing for this club."