Paul Robinson's late own goal denied West Brom victory after Rob Earnshaw opened his Baggies goalscoring account.
Southampton were the better side early on and deservedly went in front through Anders Svensson's diving header.
But Earnshaw deflected in Jonathan Greening's header and then smashed in a second before Alan Blayney denied Zoltan Gera from a 25-yard free-kick.
Neil Clement, Greening and Earnshaw all went close to sealing it before Robinson turned in Svensson's cross.
Southampton, buoyed by their 2-2 draw at Arsenal last week, were quicker to settle and Darren Kenton had a shot parried by Russell Hoult from a tight angle.
Managerless West Brom were lacking in invention and were made to pay when Svensson swooped after a good early pass from Telfer.
But Saints were stung almost immediately when Greening got on the end of Gera's cross and his header was deflected in by Earnshaw.
Third-choice goalkeeper Alan Blayney, in because of a knee injury to Antti Niemi in training, should have done better with that effort and eight minutes later when Earnshaw grabbed his second.
A long ball down the middle caused disarray in the home defence and, after Greening's cross was partially cleared by Rory Delap, the Wales striker doubled his Albion goal tally.
The game had turned around completely and Gera powered in a free-kick towards the top corner but Blayney redeemed himself by tipping the ball over.
Southampton were booed off at half-time and a dismal start to the second period could have left them on the end of a heavy defeat.
Blayney did his confidence a power of good when he blocked a fierce drive from Clement.
It should have been 3-1 a minute later when Blayney parried a tremendous Greening strike and Earnshaw missed a great chance to complete his hat-trick with an overhead kick from three yards.
Kevin Phillips gave the besieged Saints some hope but he could not get enough on his glancing defender, which drifted harmlessly wide.
Hoult failied to hold a long-range shot from Svensson and Delap headed over an inviting Telfer cross as the home side pressed in the closing stages.
Saints snatched an equaliser when Svensson's cross was diverted past Hoult by Robinson and Delap had a great opportunity to secure an unlikely victory but he could not get enough power on his shot.
Southampton: Blayney, Kenton, Lundekvam, Jakobsson (McCann 60), Van Damme, Nilsson (Prutton 84), Telfer, Delap, Anders Svensson, Phillips, Blackstock (Ormerod 63). Subs Not Used: Poke, Higginbotham.
Booked: Blackstock, Kenton.
Goals: Anders Svensson 28, 87.
West Brom: Hoult, Scimeca, Moore, Purse, Robinson, Gera (Koumas 83), Johnson, Clement, Greening, Kanu (Horsfield 77), Earnshaw. Subs Not Used: Albrechtsen, Kuszczak, Dyer.
Goals: Earnshaw 29, 37.
Ref: S Bennett (Kent).