Boy held over fake Potter file.
He took two weeks to rewrite Harry Potter and the Deathly Hallows after it went on sale last month.
The pirate copy was downloaded by thousands of French readers, who must wait until October 27 for the official version.
Police said: "It was such a good translation we thought we would find a professional who had done it to make money.
"Instead, we found a teenager working in his bedroom and offering to download it for free."
Author JK Rowling has expressed "deep concern".
French publisher Gallimard added: "This could seriously affect sales."
The boy, from Aix-en-Provence, could face a large claim for damages.