Match ends, Monaco 1, Valenciennes 2.
Record signing Radamel Falcao missed a penalty on his return to the side as Monaco failed to take their chance to go top of Ligue 1.
The £50m striker, who signed from Spanish side Atletico Madrid in May, came on as sub only to see his spot kick saved by Nicolas Penneteau.
Valenciennes had taken a 2-0 lead courtesy of Eric Abidal's own goal before the break and David Ducourtioux.
James Rodriguez scored a late consolation for Monaco who stay second.
It was Monaco's first home loss of the season as the principality side fell behind when France defender Abidal put into his own net after Danijel Subasic had parried a cross.
Valenciennes, who are now third from bottom, scored their second when Ducourtioux beat Subasic from close range following a quick counter attack shortly after half-time.
Colombia striker Falcao entered the fray after three weeks out with a thigh strain but he was left frustrated when his penalty was saved and Rodriguez's effort could not prompt a comeback.
- 24RaggiSubstituted forFerreira-Carrascoat 64'minutes
- 22AbidalBooked at 72mins
- 8João Moutinho
- 29RivièreSubstituted forFalcaoat 45'minutes
- 23MartialSubstituted forOcamposat 56'minutes
- 25MaterBooked at 68mins
- 21CissBooked at 52mins
- 4Enza Yamissi
- 24KagelmacherBooked at 81mins
- 27MeliksonSubstituted forSaezat 59'minutesBooked at 86mins
- 10DosseviSubstituted forMassampuat 83'minutes
- 3MasuakuBooked at 15mins
- 26Da Silva
- Olivier Thual
- Shots on Target
Second Half ends, Monaco 1, Valenciennes 2.
Yannick Ferreira-Carrasco (Monaco) wins a free kick in the defensive half.
Foul by David Ducourtioux (Valenciennes).
João Moutinho (Monaco) wins a free kick in the defensive half.
Foul by Kama Massampu (Valenciennes).
Offside, Monaco. Jérémy Toulalan tries a through ball, but Mounir Obbadi is caught offside.
Foul by Jérémy Toulalan (Monaco).
Tongo Hamed Doumbia (Valenciennes) wins a free kick in the attacking half.
Offside, Monaco. Falcao tries a through ball, but James Rodríguez is caught offside.
Lucas Ocampos (Monaco) wins a free kick on the right wing.
Foul by Arthur Masuaku (Valenciennes).
José Saez (Valenciennes) is shown the yellow card for a bad foul.
João Moutinho (Monaco) wins a free kick in the attacking half.
Foul by José Saez (Valenciennes).
Corner, Monaco. Conceded by Arthur Masuaku.
Corner, Monaco. Conceded by Saliou Ciss.
Foul by Mounir Obbadi (Monaco).
Tongo Hamed Doumbia (Valenciennes) wins a free kick in the defensive half.
Goal! Monaco 1, Valenciennes 2. James Rodríguez (Monaco) header from the centre of the box to the bottom left corner. Assisted by Ricardo Carvalho with a headed pass following a corner.