Prosecutor Diane Ngomsik told AFP news agency that the volunteer firefighter, 18, had initially confessed to “setting fire to twigs with a lighter and petrol” before retracting it.
BFMTV said a second 18-year old man had also been placed under formal investigation – a stage in French criminal procedure which may lead to formal charges and a subsequent trial.
The Fontainebleau forest blaze partially closed the country’s main north-south motorway since it started last Sunday.
President Macron said 10% of the forest had been burned, and hailed the fact that there had been no victims so far.
The forest is home to the Palace and Park of Fontainebleau, a Unesco World Heritage site since 1981.
France has registered nearly 11,000 fires since the beginning of the year.