The arrest came during a crackdown by the Chinese government on "Internet rumors," Britain's Guardian newspaper reported Friday.
The 16-year-old boy from northwest Gansu province was identified only as Yang.
Authorities said he was arrested after accusing local police of a coverup about the body of a man found outside a karaoke parlor earlier this month.
Police said the man died from a head injury after falling from a high place. Yang wrote that the man had been murdered.
In his post, Yang said, "The karaoke parlor boss is an employee of the court and there's been a conflict between the police and the masses -- they beat up the dead man's family members."
The youth, whose Internet comments were deleted, was accused by authorities of "fabricating facts" and "picking quarrels and provoking disputes."
Officials said he could face up to three years in prison if his post is found to have received more than 5,000 views or was forwarded more than 500 times.