Police caught the boy in the act on Tuesday night in the suburb of Aubagne and he confessed to setting nearly 20 others around the southeastern city that together razed some 300 hectares (740 acres) of land.
In the most serious incident, 600 firefighters had to battle a blaze near Aubagne that sparked major rail disruption, blocking more than 3,000 passengers in stations in Marseille, Nice and Toulon.
No homes were damaged in the fire, which he started in a woodland area dotted with houses, but a nearby holiday centre had to be evacuated.
A psychological evaluation found that the boy suffered from a “fascination with fire”, Marseille prosecutor Xavier Tarabeux told reporters.
Requesting that the boy be placed in juvenile custody, Tarabeux said the boy told investigators he carried out the June-September spree out of “boredom”.
A series of forest fires burnt thousands of hectares of woodland in southeastern France and the island of Corsica over the summer.