George Boateng's deflected goal took Middlesbrough to within one point of European football and dealt a huge blow to Tottenham's own Uefa Cup ambitions.
The Dutchman rifled home from the edge of the area after 11 minutes and a touch off Ledley King left stand-in goalkeeper Radek Cerny flat-footed.
Szilard Nemeth hit the bar and Stewart Downing twice went close for Boro.
Spurs made more of a game of it after the interval but the hosts always looked the more likely to score.
They took a stranglehold on the game after coming out of the blocks fast with Nemeth hitting the woodwork on two minutes before Downing flashed a free-kick over.
Boro built on that positive start and their crisp tempo and controlled fluency left Tottenham flailing in the first half.
The goal was a case in point.
Tottenham lost possession in Boro territory and were immediately back-pedalling as a succession of passes and fluid movement cut through their defences.
Nemeth failed to connect with Boudewijn Zenden's inviting low pass from the left, but the Slovak had the presence of mind to wait for support to arrive and pull an equally inviting pass back into the box.
The on-rushing Boateng pounced, hammering a fierce shot into the net with the help of a slight deflection off King.
That Tottenham improved after the break came as no surprise, they could not have played any worse.
Robbie Keane's introduction at the expense of the anonymous Jermain Defoe in attack helped and the Irishman soon tested Mark Schwarzer with a volley on the turn.
But while the game opened up they were unable to create a telling breakthrough.
Instead it was the hard-working Jimmy Floyd Hasselbaink who came closest to scoring with a fizzing shot that stung Cerny's palms.
Downing also tried his luck from distance and in the closing stages Boro were once again running the show as they had done in the early exchanges.
Hasselbaink blazed over in the dying moments before Nemeth curled a shot wide with the last kick of the game.
Tottenham had surrendered the initiative from the whistle and rarely looked like regaining it against a well organised team.
And after this result it looks likely that Spurs will be unable to regain seventh place and a Uefa Cup spot from Boro.
Middlesbrough manager Steve McClaren
"We had more experience out there in a must-win game. They know how to win and they did the job.
"It was the biggest game of the season and experience won in the end.
"We should have scored more on the break and had plenty of chances, but we defended magnificently and if we keep another clean sheet we're in Europe."
Tottenham manager Martin Jol
"It's always difficult when you go away from home, but it was a soft goal and that was disappointing.
"You then have to play in a different style and take the initiative, but we weren't good enough to score two goals.
"They tried to hit us on the break and could have scored two or three goals. We are one of the younger sides in the Premiership and it showed today."
Middlesbrough: Schwarzer, Parnaby, Southgate, Ehiogu, Queudrue, Parlour, Boateng, Zenden (Doriva 78), Downing, Hasselbaink, Nemeth.
Subs Not Used: Reiziger, Cooper, Morrison, Knight.
Goals: Boateng 11.
Tottenham: Cerny, Edman, King, Dawson, Kelly, Reid (Mido 69), Carrick, Davis, Davies, Kanoute (Ziegler 83), Defoe (Keane 58).
Subs Not Used: Bunjevcevic, Fulop.
Booked: Davis, Carrick, Mido.
Ref: D Gallagher (Oxfordshire).