The 31-year-old has established himself as a firm fans' favourite at St Mary's since joining from Bristol Rovers in August 2009.
Lambert has scored 101 times for Southampton and his 13-goal haul this season makes him the top English scorer in the Barclays Premier League.
The striker's contract had been due to expire at the end of next season but has now been extended by a further two years.
"I'm over the moon to sign with Southampton for another three years," Lambert told the club's official website, www.saintsfc.co.uk.
"I am already enjoying myself massively at this club and the aim now is to stay on for another three years and help push the club on further than where it already is.
"There is a lot of ambition in this club for the next few years and that is one of the main reasons that I have signed the contract - and that is why I want to end my career here.
"The core of players who have been together for two or three years are going to be here for another three or four years and that is a really good feeling.
"We all have the same ambition and [executive chairman] Nicola Cortese's ambition for this club drives us all on as well, so it really helps the club with where we are now and it will help with where we want to go."
- Sports & Recreation