The French champions maintained their eight-point lead over second-placed Monaco, who had Dimitar Berbatov to thank for a 3-2 win at Olympique Lyon.
PSG have 70 points from 29 matches after Ibrahimovic surpassed Argentine Carlos Bianchi's PSG record of 39 goals scored in all competitions in a single season (1977-78).
St Etienne stayed fourth on 48 points, five points adrift of third-placed Lille who occupy the Champions League third qualifying round spot.
Laurent Blanc's PSG side predictably dominated possession but the first clear chance was for St Etienne when Salvatore Sirigu tipped away Franck Tabanou's spectacular rising shot on the turn from outside the box in the 13th minute.
Ibrahimovic, however, opened the scoring in the 16th minute at the end of a bizarre move.
The Swede played the ball into Blaise Matuidi, who was flagged offside but referee Lionel Jaffredo ignored the linesman's call and the ball fell for Ibrahimovic who found the back of the net with a low cross shot.
TV replays showed that Matuidi had gained possession from St Etienne defender Mustapha Bayal Sall's back pass.
Brandao was lucky to escape a red card after elbowing Yohan Cabaye in the face as PSG increased the pressure, coming close twice through Ibrahimovic and Edinson Cavani.
Both combined perfectly in the 42nd, and Ibrahimovic stabbed in a close-range volley with the outside of his foot from the Uruguay forward's perfect cross.
St Etienne had a clear chance to reduce the arrears after the break but Benjamin Corgnet inexplicably shot wide from point-blank range.
Earlier on Sunday, Dimitar Berbatov scored one goal and set up two more in Monaco's win at Gerland.
Berbatov created Valere Germain and James Rodriguez's first-half goals before chipping in the third.
Lyon stayed fifth on 45 points.
Germain put the visitors ahead after four minutes from close range after Berbatov had tapped the ball into this path.
The Bulgarian was at it again in the 27th minute, setting up Rodriguez from close range again.
But Lyon did not give up and reduced the arrears five minutes later as Jimmy Briand latched onto Miguel Lopes's pass to beat Danijel Subasic.
Rodriguez repaid Berbatov's favour by setting up the former Tottenham and Manchester United striker for Monaco's third, a fine chip past Anthony Lopes.
Remi Garde's team, however, were back in contention with 12 minutes left as Briand scored his sixth league goal of the season with a volley.
But it was not enough as the Monaco defence, perfectly marshalled by Ricardo Carvalho, held on.
- Sports & Recreation
- Dimitar Berbatov
- Blaise Matuidi