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.
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."
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.
Keeper Steve Mandanda was sent off in the 25th minute after bringing down Lille's Gervinho and Cabaye converted the resulting penalty to equalise.
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.
