Julian Draxler, Edinson Cavani, Angel di Maria and youngster Moussa Diaby all found the net at the Parc des Princes to allow PSG to make it five wins from five games at the start of their defence of the French title.
It was a fine win for PSG, who have already scored 17 goals in the league this season, and Thomas Tuchel is the first coach to win his first five league games in charge of the club.
However, there is little doubt that PSG will face a far tougher examination at Anfield in their opening Champions League group outing than they did here against Saint-Etienne.
French Ligue 1 to compete with European counterparts
Mbappe was absent as he began a three-match ban handed down following his red card in the win at Nimes just before the international break, and Neymar sat alongside him in the crowd, rested after playing for Brazil in a friendly in the United States in midweek.
Being without the two most expensive players in the world was not a problem for the reigning champions, with Tuchel able to give Draxler his first start of the season while midfielder Marco Verratti appeared in a Ligue 1 game for the first time since March.
There was also a debut for Spanish left-back Juan Bernat following his recent move from Bayern Munich, and while the visitors started well enough, PSG never looked back after taking the lead midway through the first half.
Verratti's ball over the top found Draxler in the box, with the German midfielder controlling the ball with his head and then nodding it over Saint-Etienne goalkeeper Stephane Ruffier.
The Saint-Etienne players were wearing shirts carrying the name of William Gomis, a former youth player at the club who was shot dead in the south of France earlier this week.
PSG's second goal arrived just after half-time, with Cavani converting a penalty, his third goal in as many games this season, after he had been chopped down in the box by Loic Perrin.
Neymar to stay in Paris
The hosts went on to add two more goals late on. Diaby, the 19-year-old who was sent on at the start of the second half in place of Lassana Diarra, helped set up Di Maria to lash a shot into the roof of the net for the third.
Then Diaby rounded off the scoring after Cavani's swipe at a Draxler cutback had turned into an assist for the teenager.
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