The attacking midfielder, who turns 25 next month, follows goalkeeper Kelvin Davis, striker Rickie Lambert, as well as midfielders Morgan Schneiderlin and Jason Puncheon who have all signed new contracts in recent months.
Lallana struck his third top-flight goal in last weekend's win over Reading, which took him to a career total of 50 goals for the club in just over 200 first-team appearances.
Having worked his way up from the youth ranks at Southampton, Lallana has helped them climb from the third tier to their current position of 11th in the Premier League.
"I'm absolutely delighted," he 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, who was called up to the senior England squad for the first time for the 2014 World Cup qualifier against Ukraine in September, has struggled with injury this season.
However, Southampton manager Mauricio Pochettino has relied on him to lead the team in their battle against relegation following their return to the top flight this term.
"He is not only important because of how he plays, but also with his behaviour off the pitch and how he transmits to other players," said the Spaniard.
"Adam is a home-grown player so he bleeds the colours of this team," he added.
