Zlatan Ibrahimovic, making his injury return, scored a hat-trick and strike partner Edinson Cavani also netted as the big-spending Parisians ran riot in the capital to move up to second.
"It's normal when you add a very good player that you play better," said PSG coach Laurent Blanc.
"We are therefore better with him, his presence is important."
But they were given a huge helping hand by Saint-Etienne goalkeeper Stephane Ruffier who failed to control a back pass on 24 minutes, turning the ball into his own net as he tried to the steer it on to his right foot but instead pushed it over the line.
Ibrahimovic, making his return from injury, notched his first four minutes before the break with a firm header from Maxwell's left-wing cross.
He doubled his tally as he ran on to a wayward back pass on 62 minutes before rounding Ruffier and slotting into the empty net.
A minute later Cavani lashed in an unstoppable drive from distance that left Ruffier rooted to his line.
Ibrahimovic completed his hat-trick on 72 minutes after beating a static offside trap and slotting the ball past Ruffier at his near post.
St Etienne coach Christophe Galthier paid tribute to the Sweden star.
"Our opponents were superior in every department, showed great determination and had their leader back," he said.
"I had said I'd like to see Ibra but but I didn't want to see him score."