The Sweden international's first-half brace put a huge dent in les Verts' hopes of playing in the Champions League next season
Each of the Sweden international's strikes on Sunday came in the first half, with his brace enabling the Ligue 1 leaders to restore their eight-point lead over Monaco at the top of the table.
Ibrahimovic has now scored 25 times in the league alone and had opportunities to register his third hat trick of the campaign.
PSG had to withstand pressure in the second half at the Parc des Princes, with Salvatore Sirigu doing well to keep out Yohan Mollo and Moustapha Sall before Josuha Guilavogui wasted a golden opportunity.
Saint-Etienne was unable to find a way through and missed the chance to close the gap on third-placed Lille, meaning it remains five points off the UEFA Champions League places with nine games remaining.
The visitors created the first clear-cut opportunity after 13 minutes. Franck Tabanou received the ball on the edge of the penalty area and was allowed to turn as the PSG defense stood off, but the attacking midfielder's left-footed shot was pushed away by Salvatore Sirigu.
PSG soon made its opponent pay as Ibrahimovic opened the scoring a minute later, although the goal was not without controversy.
Blaise Matuidi, who appeared to be offside, saw a shot blocked after being played in by Ibrahimovic. The loose ball ran to the Sweden international, who swept home without breaking his stride.
Ibrahimovic saw a free kick deflect wide off the Saint-Etienne wall and missed the target again when mis-timing a volley from six yards out.
He made no mistake on 41 minutes, though, meeting an Edinson Cavani cross to beat Ruffier for a second time.
Saint-Etienne came out all guns blazing in the second half and went close as Mollo forced Sirigu into a close-range save, with Benjamin Corgnet unable to turn in the rebound.
Sirigu was again on hand to keep the visitors at bay on 52 minutes as he parried a shot from Sall, the unmarked defender having fired towards goal from inside the penalty area.
Saint-Etienne had another chance to get back into the game just before the hour as Guilavogui met a left-wing free kick from Mollo, only to head over from six yards out.
PSG gradually regained control thereafter and went close to adding a third when Ibrahimovic released Maxwell, but Matuidi was unable to convert the Brazilian's cross.
There was a painful moment for Ibrahimovic when he was clipped from behind by Guilavogui, but the 32-year-old was able to continue after treatment and went on to sting the palms of Ruffier with a late free kick.