Benjamin Corgnet's first two goals for the club since his move from Lorient gave Saint-Etienne the win and sent Valenciennes to their fourth defeat of the season.
The midfielder broke the deadlock in the 17th minute, with Romain Hamouma doubling their lead two minutes into the second half.
Corgnet added his second in the 53rd minute as the visitors coasted to the three points.
Maor Melikson grabbed an 80th-minute penalty for the home side, but it was mere consolation as they dropped to another defeat.
Marseille kept their strong start to the season going, but had to come from behind to snatch a point against Toulouse.
Wissam Ben Yedder gave the hosts the lead just after the hour mark, and they looked on course to record their first win of the season.
However, Lucas Mendes came to Elie Baup's side's rescue with an 87th-minute strike that earned them a point.
Sochaux's poor start to the season continues as they lost 1-0 to Nantes.
Filip Djordjevic got the only goal of the game at the Stade de la Beaujoire, converting a Banel Nicolta pass in the 76th minute.
The result leaves Sochaux bottom of Ligue 1 with two points from their opening five games, and still searching for their first win.
Ajaccio are also yet to taste victory after they came out on the wrong end of a five-goal thriller against Evian.
Brice Dja Djedje and Kevin Berigaud gave Evian the lead at half-time, with Berigaud adding a second seven minutes after the break.
Things looked comfortable for the visitors until Paul Lasne and Gadji Tallo pulled two goals back in the final nine minutes, but Evian were able to hold on and secure the win.
Mustapha Yatabare scored a late penalty equaliser for Guingamp to rescue them a point against Bastia. Wahbi Khazri had opened the scoring for the visitors but they were unable to record the victory.
However, it could have been even better for Yatabare had he not missed an injury time penalty.
Elsewhere, Montpellier and Reims battled out a goalless draw at the Stade de la Mosson.
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