The high school student from the southern city of Aix-en-Provence was picked up Monday based on a complaint from police in Paris, said Aix Prosecutor Olivier Rothe. The boy was released Tuesday after questioning.
Rothe said judicial police in Paris were conducting an investigation and that the boy could face charges for violating intellectual property rights.
The boy had apparently compiled the entire translation of J.K. Rowling's 759-page "Harry Potter and the Deathly Hallows" himself, but was not thought to have sought any commercial profit from the unauthorized work, Rothe said.
It was available online within days of the July 21 release of the English original. Police later shut down the site where the boy posted the translation.
The publishing house Gallimard, which has published the Potter series in French and is to release the official translation of "Deathly Hallows" on Oct. 26, would not comment on the case before consulting with company lawyers.
Photos: Harry Potter Fans Around The World
Marie Leroy-Lena, spokeswoman for official Harry Potter translator Jean-Francois Menard, said he is still working on the translation of "Deathly Hallows," since he only received the official English version when it was released two weeks ago.
Fans in several countries have already posted unofficial translations of the book online, including in China, where publishers fear it could lead to counterfeit books in a country where piracy is rampant.
In France, the daily Le Parisien revealed how the final installment of "Harry Potter et les reliques de la mort" — as it is called in French — ends, in a small article which it printed upside down, so readers wouldn't have the ending thrust on them unknowingly.
Worldwide, the "Potter" books have sold more than 325 million copies, have been translated into at least 64 languages, and have been spun off into a hit movie series.