The 23-year-old defender has been linked with a move to the Premier League at the end of the season, and Arsenal and Manchester City are thought to be battling it out for his signature.
But reports in France claim Ligue 2 club Monaco have now entered the race for N'Koulou.
The French giants currently sit top of the Ligue 2 table, and have been linked with a summer spending spree should they secure promotion back to the summit of French football.
23-year-old N'Koulou has impressed for Marseille this term, and the report claims Arsene Wenger and Roberto Mancini have both had scouts track his progress.
A £6m bid could be enough to praise him away from the Stade Velodrome this summer, but the intense interest in the defender could prompt a bidding war and drive his price higher.
The report claims the three Premier League teams must move quickly if they're to beat off the competition for N'Koulou, with Arsenal and City among the favourites.
But Newcastle, with their extensive scouting network in France, could hold the upper hand by virtue of their existing French speaking playing squad.
Mapou Yanga-Mbiwa, Yohan Cabaye, Mathieu Debuchy, Moussa Sissoko and Papiss Cisse are all French speaking, and all but Cisse have played in Ligue 1 in recent seasons.