The French police anti-graft department Oclciff started the investigation in November last year, sources revealed on Friday, following a legal complaint by trade unionist Jean-Luc Touly, who is also the president of an anti-corruption campaign.
Romanian investigators suspect Apa Nova, which supplies water to Bucharest, of paying more than 12 million euros in bribes to win contracts between in 2008 and 2015.
The price of water in the Romanian capital has soared 125 percent in the same period, according to local media.
Pointing out that Veolia, which owns 78 percent of Apa Nova, "seems to be the principal beneficiary" of the fraudulently obtained deals, Touly said it was difficult to believe that it did not know about the alleged bribery in his complaint.
Nine people have been prosecuted in Romania, two of them French - Apa Nova's former director Bruno Roche and his successor Laurent Lalague.
The others are four Romanian members of Apa Nova's management, politician Vlad Moisescu, former mayoral adviser Costin Berevoianu and businessman Ovividiu Semenescu.