Leandro Trossard rocked Tottenham's top four challenge as the Brighton forward sealed a shock 1-0 win for the Premier League strugglers on Sunday.
Jose Mourinho's side were hoping to get back on track after losing to Liverpool on Thursday, but instead suffered a damaging second successive defeat.
Trossard netted in the first half at the Amex Stadium and, with Tottenham's Harry Kane sidelined with injury, the visitors lacked the quality to score an equaliser.
Tottenham are languishing in sixth place, with just two wins in their past nine top-flight matches.
They trail six points behind fourth-placed Leicester, albeit with a game in hand, after only their second defeat in 10 away league games this season.
The title bid that looked possible for Tottenham in December is fading fast as leaders Manchester City are now 11 points clear of the north Londoners.
A victory against seventh-placed Chelsea on Thursday is essential to halt Tottenham's sudden decline away from the Champions League places.
Fourth-bottom Brighton moved seven points clear of the relegation zone after extending their unbeaten league run to three matches.
The Seagulls were the only team in the top four divisions in England without a home league victory this season, but they finally ended that dismal sequence at the 11th attempt.
Gareth Bale was given only his second league start since returning for his second spell at Tottenham.
The on-loan Real Madrid forward replaced Kane, who suffered his latest in a long list of ankle problems against Liverpool, but once again he failed to impress.
Brighton nearly made the perfect start when Pascal Gross hit the outside of the post after good work from Alexis MacAllister.
MacAllister fired from long range soon after as Brighton's vibrant start pushed Tottenham on to the back foot.
The hosts deservedly took the lead with a flowing move in the 17th minute.
MacAllister picked out Gross and his cutback found Trossard for a clinical, close-range finish.
Son Heung-min led Tottenham's first incisive raid when he found Steven Bergwijn on the edge of the penalty area, but the forward dragged his shot wide.
Bale was subdued in attack but the Wales star came in useful at the other end when he nodded clear just as Lewis Dunk's header was flying towards the top corner.
Mourinho sent on Carlos Vinicius in place of centre-back Davinson Sanchez at half-time.
The on-loan Benfica striker made an immediate impact with a towering leap to meet Bale's corner with a header that was blocked by Brighton keeper Robert Sanchez.
Tottenham still could not find their rhythm and, after Hugo Lloris saved well from Ben White's deflected effort, Mourinho hauled off the disappointing Bale and introduced Lucas Moura.
It made no difference and Brighton could have doubled their lead when MacAllister shot into the side netting from a tight angle before Neal Maupay headed wide from Solly March's cross.
Vinicius offered a glimmer of hope with a powerful effort from the edge of the area that Sanchez pushed away, but it was too little too late for spluttering Tottenham.
