Elsewhere, Lisandro Lopez struck his second goal of the game deep into stoppage-time as Olympique Lyon rose to third with a dramatic 2-1 win at Montpellier.
Montano struck for hosts Rennes in the 64th minute after Nampalys Mendy handled the ball.
"It feels good. I have been waiting to score a goal at home for a long time," Montano told the Foot Plus TV channel after a dull match containing few chances.
"It was a bit complicated (because) we were tired after our Wednesday game against Marseille."
Three days ago Jires Ekoko missed a fifth-minute penalty for Rennes in their 0-0 draw at Marseille.
"I am disappointed because we put in a lot of effort," said Monaco defender Sebastien Puygrenier.
LOPEZ DOUBLE
In Montpellier, man of the match Lopez opened the scoring for Lyon in the fifth minute.
Emir Spahic levelled for the home team when his header fooled France goalkeeper Hugo Lloris 10 minutes from time.
Lopez had the last word, though, grabbing the winner four minutes into stoppage-time.
"Our state of mind is positive," said Lloris. "We remained focused and we stuck together. We are getting better and better."
After a lacklustre start to the season during which coach Claude Puel was lambasted by his own supporters, Lyon are right back in the title race one point off the lead.
Sochaux are sixth after beating Valenciennes 2-1 and Toulouse occupy eighth place after a 1-0 win over second from bottom Caen who registered their sixth defeat in a row.
