Yang was released after a week at a police detention centre in Zhangjiachuan, where he had been held on charges of "causing trouble". He had been picked up by police last week after he wrote an online post in which he raised doubts about a local police investigation into the death of a karaoke bar manager.
Yang is the youngest known microblogger to be detained in an official, nationwide anti-rumour campaign aimed at reining in free speech.
Police detained the teenager for his online post, which had been shared more than 500 times.
His case galvanised many lawyers on the mainland who began calling for his release.
"If China's netizens unite, who can stop them?" noted Zhou Ze, a well-known Beijing-based lawyer, in an online post.