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 goals 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.
Source: PA
Source: PA