The pressure was on Bale to deliver after a brilliant week for the Welshman, but for most of the game he disappointed.
The 23-year-old, who won the PFA Player of the Year, Young Player of the Year, and FWA Footballer of the Year awards this week, was marked out of the game for most of what was largely a dull and uninspiring match.
Bale flitted between positions, his touch was often poor, and he offered little to help Spurs on the attack.
That was until the 86th minute. The forward found space 25 yards out, picked the ball off substitute Lewis Holtby and unleashed a peach of a shot which beat the out-stretched Artur Boruc.
The strike brought his tally for the season to 30 - double his previous best - and also meant Bale became the first Tottenham player since Jurgen Klinsmann to register 20 league goals in a season.
The victory piled the pressure on Arsenal and Chelsea, in the battle for top-four spots in the Barclays Premier League, ahead of Spurs' huge clash against the Blues on Wednesday night.
But it was a cruel blow for Southampton, who deserved at least a point from the game.
Hugo Lloris had tipped a brilliant Rickie Lambert free-kick in the first half on to a post, and the Saints dominated most of game, but they remained far from safety following Wigan's win at West Brom.
After a half-hour delay due to a chemical spillage on the M25, Tottenham started brightly, going on the attack from the first whistle. Jermain Defoe almost opened the scoring in the fifth minute when he drove into the side-netting.
The home side's high line left them vulnerable, though, and Southampton took full advantage.
Lambert sent Nathaniel Clyne through in the 10th minute with a clever chip, but the right-back hammered a volley inches wide from 10 yards.
Spurs boss Andre Villas-Boas spent the opening 20 minutes on his feet, frantically motioning to his team, who were second best.
Southampton broke again through Lambert, who fed Guly Do Prado, but his pull-back was cut out by Michael Dawson before Jay Rodriguez could tap in.
Rodriguez went close from the resulting corner as Southampton continued to press.
The pressure got to Villas-Boas, who turned to some dissenting members of the crowd and told them to curb their criticism.
Defoe offered a glimmer of hope when he raced past Jos Hooiveld, but Jose Fonte came to the rescue with a brilliant sliding tackle.
With just over half an hour gone, Tottenham's chances of victory took a blow when Mousa Dembele hobbled off with an injury to be replaced by Holtby.
Villas-Boas shifted Bale out to the left flank, but he was still being stifled by tight marking. His only first-half contribution was a stoppage-time header which sailed well over Boruc's goal.
Ten minutes earlier Saints almost took the lead through Lambert, whose low free-kick was tipped on to the woodwork by a diving Lloris.
Southampton remained on top in the opening stages of the second half.
A long-range free-kick from Lambert whistled past Lloris' left-hand post while Kyle Walker denied Rodriguez with a crucial header.
Villas-Boas looked to Emmanuel Adebayor for inspiration, the Togolese coming on for an injured Aaron Lennon with half an hour left.
Tottenham still failed to test the Southampton goalkeeper, though. Clint Dempsey got on the end of Adebayor's knock-down, but blazed over.
Villas-Boas changed things around again by introducing Gylfi Sigurdsson for Dempsey with 20 minutes left.
Boruc offered Tottenham a way in soon after when he flapped at Tom Huddlestone's cross, but Jan Vertonghen's header was cleared off the line. Spurs thought the ball struck a Southampton hand, but referee Mark Clattenburg waved play on.
The hosts' hopes of salvaging a win looked to be slipping away until the 86th minute, when that man Bale collected the ball off Holtby and fired a 25-yard shot past a sprawling Boruc.
Bale had said earlier this week that he would mute his celebration out of respect to his former club, but the 23-year-old could not resist, racing over to the corner before being mobbed by his delighted team-mates.
Spurs held on for four agonising minutes of stoppage time, before scenes of joy erupted following the final whistle.
The scenes annoyed Saints boss Mauricio Pochettino, who had to be restrained by Villas-Boas after an altercation with Spurs assistant Steffen Freund.