Marseille are in second place after 17 rounds, with third-place Monaco 12 points adrift of defending champions PSG, who have beaten both sides.
Dembélé scored in the 13th minute of Sunday's match and the France winger netted from the penalty spot 10 minutes later after a video review spotted a handball from Léo Pétrot on Achraf Hakimi's cross.
PSG have been linked with a move for Georgia winger Khvicha Kvaratskhelia from Napoli, but it was another Georgian who scored for 16th-place Saint-Etienne as Zuriko Davitashvili neatly curled in a free kick midway through the second half.
Despite their unbeaten league record, PSG again looked nervous and came close to conceding an equaliser.
Earlier, Deiver Machado scored late as seventh-place Lens won 2-1 at struggling Le Havre to move one point behind Lyon in sixth.
The Colombia defender headed powerfully home from Poland winger Przemyslaw Frankowski's cross in the 77th minute.
Central defender Abdukodir Khusanov was left out of the Lens squad amid reports of an imminent big-money transfer to Premier League defending champions Manchester City.
The 20-year-old joined Lens on a four-year contract last season, becoming the first Uzbek to play in Ligue 1.
Veteran winger Andre Ayew put Le Havre ahead in the eighth minute.
Striker Goduine Koyalipou equalised in the 28th with a header on debut after only joining on Thursday.
Le Havre have lost five straight league games and are in 17th spot.
Improving Strasbourg climbed into ninth place with a 2-1 win at Toulouse in a match where video reviews affected both sides, and rock-bottom Montpellier had two players sent off near the end of a 3-1 home loss to Angers.
Dutch striker Emanuel Emegha scored two first-half goals for Strasbourg.
Toulouse scored from Ismaël Doukouré's own-goal in the 35th and, in the 75th, striker Zakaria Aboukhlal prepared to take a penalty to make it 2-2 after being fouled. However, the referee changed his mind following a video review and instead awarded Toulouse a free kick right on the edge of the area.
More frustration followed for the home side moments later.
Toulouse thought they had equalised when Frank Magri side-footed in Aboukhlal's pinpoint cross from the left. But VAR intervened again to rule the goal out because defender Mark McKenzie was narrowly offside.
Forward Esteban Lepaul netted twice as Angers moved up to 13th place. He scored in each half, either side of a penalty from Montpellier midfielder Téji Savanier.
Montpellier lost their composure in the closing stages when defender Boubakar Kouyaté and captain Jordan Ferri were sent off within one minute of each other.
Midfielder Zinedine Ferhat added the third goal for Angers in the ninth minute of stoppage time.
