Ibrahimovic, who has now netted 17 goals in 14 league appearances, helped second-placed Paris Saint-Germain cut Olympique Lyon's lead to two points before the seven-times French champions travel to basement side Nancy on Wednesday.
PSG have 32 points from 17 games and lead third-placed Olympique Marseille, who play at Bastia on Wednesday, by three points.
"Our confidence has been growing," PSG midfielder Blaise Matuidi told Canal Plus. "We beat a team who were unbeaten at home, and in style."
"We're confident. It is surely going to be a spectacular match," said Matuidi.
Valenciennes had the first chance after nine minutes when Matthieu Dossevi's sweet low shot went just wide.
PSG keeper Salvatore Sirigu then did well to punch away Nicolas Isimat-Mirin's header as the visitors struggled to contain Valenciennes.
Pastore found Jeremy Menez with a clever back heel and the France forward's shot was deflected into the path of Ezequiel Lavezzi, whose attempt was steered past goalkeeper Nicolas Penneteau by PSG's Sweden striker.
The visitors' Dutch centre-back Alex was replaced by Mamadou Sakho on the stroke of half-time after picking up an injury.
Ibrahimovic doubled his tally five minutes into the second half when he poked the ball home after Penneteau parried Pastore's volley into his path.
Ibrahimovic, who had already scored five doubles in the league this season, completed his treble four minutes later, tapping in Menez's cross from the left.
Lavezzi, who has been approaching the form that made him a fans' favourite at Napoli, rounded off the victory in the 83rd at the end of a solo run after being nicely set up by Marco Verratti's through ball.
