Bernardo Silva opened the scoring in the 47th minute with apowerful strike from outside the area, before Andrea Raggi settled a low-key contest with a close-range effort in the 75th.
The win moves Monaco into second place on 58 points, two ahead of Lyon, who were held to a 1-1 home draw by Nice -- a further two points back in fourth -- on Friday. The principality side are 28 points adrift of champions Paris St Germain.
Marseille, who pulled a goal back when Michy Batshuayi was set up by Steven Fletcher in stoppage time, are just six points clear of the drop zone in 15th.
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"The important thing is that we're back in second place," Monaco midfielder Bernardo Silva told Canal Plus. "Now we need to hold onto it."
Elsewhere, 16th-placed Montpellier eased their relegation fears with a 2-0 victory at injury-hit Nantes, while Guingamp won 3-0 at Breton rivals Stade Rennes, all the goals coming in 14 second-half minutes.
(Reporting by Ed Dove; editing by Neil Robinson)
