The win gave PSG 36 points from 17 games, level with Montpellier, who went down 1-0 at Valenciennes to Foued Kadir's second-half strike but stayed top on goal difference.
Olympique Marseille, who qualified for the Champions League last 16 earlier this week, were left in eighth place 11 points off the pace after a disappointing goalless draw at home to improving Girondins Bordeaux.
"We played well in the first half but could not keep it up after the break," PSG defender Mamadou Sakho told French TV channel Foot Plus. "But we won, so we're satisfied."
Valenciennes dominated and were rewarded a minute after the break when Geoffrey Jourdren parried David Ducourtioux's fierce shot into the path of Foued Kadir who poked the ball home.
"We failed to play our usual football," Giroud, who scored one goal and set up three more last weekend in an impressive 4-0 hammering of Lorient, told Foot Plus.
"They [Valenciennes] played it like a cup game while we were not as sharp as usual. It's a setback. Maybe, when you see you can't win a game you should just try not to lose it."
