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Djibril Sidibe curled in the opener after Keita Balde shot was blocked and Thomas Lemar -- the target of a bid by Arsenal in the summer -- scored on 32 minutes after weaving through the St Etienne defence.
Fabinho, who told the club this week he wants to leave, tapped in the third in the second half before Keita put the icing on the cake with a piercing shot on 60 minutes at Stade Geoffroy-Guichard, where there were row after row of empty seats.
The victory put Monaco six points behind Paris Saint-Germain, who play at Rennes on Saturday.
Saint-Etienne are hovering just above the relegation zone.
"It was a great night," Monaco coach Leonardo Jardim said.
"We finished our chances well and we could have scored more goals if we had been more clinical."
