The long-serving midfielder, who came through the club's academy, scored in last weekend's 2-0 victory over Reading, his third of the season and 50th for the club in total.
He follows Kelvin Davies, Rickie Lambert, Morgan Schneiderlin and Jason Puncheon in signing a new deal with the club in recent weeks.
"I'm absolutely delighted," Lallana told the club's 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."
Lallana has made more than 200 first-team appearances for the Saints, having helped them rise from League One to the Premier League.
"I've been at the Club since I was 12 and - being honest - I can only see my future lying with Southampton Football Club," he added.
"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."
- Sports & Recreation