Li Hai was allowed on Sunday to leave Beijing's Liangxiang prison early in the day after completing a nine-year sentence, according to the prison official.
"He left the prison at about 9:00 am (0100 GMT)," said the official, surnamed Xiao.
Beijing police have visited Li's mother in recent days and warned her not to allow any "unanticipated events" to occur after his release, according to New York-based Human Rights in China.
Li was detained in May 1995 and later convicted of "providing state secrets to entities overseas," after giving information to foreign media and rights organizations on the government's arrests and trials of other dissidents.
A graduate philosophy student at Peking University in the late
1980s, Li had already spent 18 months in prison after taking part in the Tiananmen protests that were crushed on 4 June 1989.
While in Liangxiang Prison, he was subjected to severe physical and mental abuse, with most of his sentence spent in solitary confinement or under close monitoring, according to Human Rights in China.