The 24-year-old Saints captain, who joined the club's academy in 2006, put pen to paper on a contract on Thursday evening.
Lallana is the fifth Southampton player to commit his future to the club after Kelvin Davis, Rickie Lambert, Morgan Schneiderlin and Jason Puncheon all signed recent deals.
"I'm absolutely delighted. To commit my future to the club for the next five years is something I'm really happy about," he said.
"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. I can't wait to get going and to continue playing for this club."
Manager Mauricio Pochettino expressed his delight at the news, adding: "I'd like to congratulate Adam on his new contract, and I'm happy for all of the supporters because this is really good news.
"We are very pleased that Adam has signed a new five-year contract. He bleeds the colours of this team."