Authorities apprehended the boy on Saturday morning as he was in the process of setting fire to a stationary car in the Mediterranean town of Herault near Montpellier.
"We are taking this matter very seriously. An investigation has been launched and will continue next week to see if this eight-year-old child is also responsible for four burned cars in the same area," said the head of the town's public safety body.
Police said that since the boy had been caught, there had been no further attacks.
"Was the boy the arsonist? Or was it a copycat attack after he saw the other four cars burned? We don't know; the police will try and work that out," said the town's mayor, Jean-Pierre Rico.
The boy was returned to his family as "prosecutors cannot take legal action" against an eight-year-old child, a judicial source said.
"However, given his profile and his family environment, we must decide whether a judge needs to order a psychiatric evaluation."