Debuchy struck four minutes into stoppage time to put second-placed Lille on 70 points with one match remaining.
They lead third-placed AJ Auxerre, who were held 0-0 by RC Lens, by two points.
A Bobo Balde own goal in injury time gave Olympique Lyon a 2-2 draw at Valenciennes.
Lyon lie fourth on 66 points and play their game in hand against Monaco on Wednesday.
The top two automatically qualify for the Champions League and the third-placed side reach the play-offs. The fourth and fifth-placed teams compete in the Europa League.
With a win at Lorient next Saturday, Lille will make sure they take part in Europe's top club competition.
"It's incredible. We got off to a really bad start," Lille keeper Mickael Landreau told French TV channel Canal Plus. "But we continued to push and somehow we won - it's amazing."
Sixth-placed Girondins Bordeaux, on 64 points, kept their slim hopes of securing a Europa League spot alive when goals by Wendel (penalty) and Benoit Tremoulinas earned a 2-0 win against Sochaux.
LAST GAME
It may prove to be Morocco striker Marouane Chamakh's last home game for 2009 Ligue 1 winners Bordeaux, with media speculation mounting that he is poised to join Arsenal.
Fans unveiled a banner saying 'Thank You Champion' as Chamakh left the pitch clapping his hands when he was replaced in the 71st minute.
"There were a lot of emotions, I have known only Bordeaux in my career," said Chamakh as he hugged his tearful parents.
Fifth-placed Montpellier, on 66 points, beat Lorient 2-1 thanks to two first-half penalties.
Marseille captain Mamadou Niang received a bunch of flowers from Lille counterpart Yohan Cabaye but the visitors offered their opponents no more early gifts, going in front when Niang fired a curling shot into the top corner on six minutes.
Keeper Steve Mandanda was sent off in the 25th minute after bringing down Lille's Gervinho and Cabaye converted the resulting penalty to equalise.
The visitors then went ahead on the stroke of half time when defender Vitorino Hilton struck with a low shot from inside the box.
Lille hit back through Tulio De Melo 10 minutes from time as Marseille's reserve keeper Elinton Andrade pulled a leg muscle.
With no keeper left on the bench, the Brazilian strapped up his injury but was left helpless by Debuchy's powerful header from a corner.
Follow FFT.com on Twitter
Join FFT.com on Facebook
