The result kept Marseille fourth on 35 points from 17 games, behind third-placed champions Monaco and second-placed Lyon on goal difference. Runaway leaders Paris St Germain have 44 points after Saturday's 3-1 home win against struggling Lille.
Germain, who moved from Monaco during the close season, headed home an 11th-minute corner and steered in a Florian Thauvin cross in the 74th after St Etienne had defender Ronael Pierre-Gabriel sent off for a professional foul.
In between the goals, Germain missed several chances as dominant Marseille hit the woodwork twice and kept raiding both flanks almost at will, with St Etienne keeper Stephane Ruffier producing a string of good saves.
Substitute Lucas Ocampos added the third with a delightfully curled shot into the top corner in the 80th minute as the home side continued to press in the closing stages.
(Writing by Zoran Milosavljevic; Editing by Neil Robinson)
