Lallana, who turns 25 in May, has impressed since being made Saints captain in the summer for the club's first campaign back in the Premier League, and scored in Saturday's 2-0 win over Reading.
He follows Kelvin Davis, Rickie Lambert, Morgan Schneiderlin, Jason Puncheon and Luke Shaw in committing his long-term future to the club, in a deal which will keep him at St Mary's until the summer of 2018.
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"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."