Chelsea peppered the Southampton goal throughout the second half but were unable to find a way past substitute goalkeeper Kelvin Davis, leading to a defeat that drops Rafael Benitez's side to fourth.
Rickie Lambert celebrated his new Southampton contract in some style today, smashing home an exquisite free-kick to secure all three points from a hard-fought encounter with Chelsea.
Making his 500th career league appearance, the Liverpudlian curled home a spectacular 25-yard free-kick to rack up his 14th league goal of the campaign - more than any other English player in the Premier League.
Saints had gone ahead through another in-form striker Jay Rodriguez, who collected a neat one-two from Steven Davis before slotting home in the 23rd minute.
John Terry was allowed to level unmarked from a corner 10 minutes later, only for Saints to hit back immediately through Lambert's fine strike.
Rickie Lambert's 35th-minute free-kick proved enough to seal a 2-1 win at St Mary's after an early opener from Jay Rodriguez had been cancelled out by a header from John Terry.
Chelsea's shock loss allowed Tottenham to leapfrog into third place with a 2-1 win at Swansea.