Montpellier, who had won their previous five league games without conceding a goal, were sunk by a late winner from home substitute Emmanuel Riviere.
"I was only the right man in the right place," Riviere told the Foot Plus channel.
Promoted Montpellier have 48 points from 25 games, three points behind Bordeaux. St Etienne are fifth from bottom, six points from safety.
The leaders are not playing this weekend as the French League agreed to postpone their game against Auxerre to give them a break before Tuesday's Champions League last-16 first leg at Olympiakos.
PSG BREAKTHROUGH
Paris Saint-Germain claimed their first league win this year when they beat 10-man Toulouse 1-0.
Striker Guillaume Hoarau converted a penalty for the home team after Albin Ebondo fouled Peguy Luyindula midway through the first half and the Toulouse defender was sent off.
PSG moved up to 12th on 33 points.
Riviere, 19, who had come on as a substitute for striker Boubacar Sanogo just minutes earlier, was on target for St Etienne in the 80th minute when he knocked in a cross by captain Blaise Matuidi at the near post.
Elsewhere, Nice ended a run of 11 games without a win by beating Lorient 1-0 thanks to a Loic Remy penalty.
Nice are fourth from bottom, six points ahead of Le Mans who won 3-1 at fellow strugglers Boulogne-sur-Mer.
Monaco, thrashed 3-0 at Racing Lens, are seventh on 39 points.
