Claudio Ranieri's men headed into the game knowing a win would take them top of the table, temporarily at least, and heap pressure on defending champions Paris Saint-Germain ahead of the latter's clash with third-placed Lille on Sunday.
However, relegation-threatened Valenciennes stole the show, with an accomplished away performance sealing only their third win of the campaign.
Valenciennes took the lead in fortuitous circumstances, Monaco defender Eric Abidal turning the ball into his own net from Arthur Masuaku's left-wing cross on the half-hour mark.
The hosts unsurprisingly dictated play in search for an equaliser, but were given a shock after 58 minutes when Valenciennes struck on the counter.
Veteran midfielder David Ducourtioux latched on to Tongo Doumbia's throughball before sweeping the ball past Danijel Subasic.
Monaco looked to have been given a lifeline seven minutes from time as Saliou Ciss' trip on substitute Radamel Falcao resulted in the award of a penalty.
Falcao's spot-kick was saved by Nicolas Penneteau, but the Valenciennes keeper was beaten from the resulting corner as James Rodriguez headed home.
Rodriguez found the net again with three minutes to play, but luck was on the visitors' side as the goal was controversially ruled offside.
