Jiang Weiping was reunited with his family after a court in Dalian, north-eastern China, commuted the final year of his six-year sentence.
The US-based Dui Hua Foundation said he was freed on good behaviour.
But it said it might be a good-will gesture ahead of an expected visit to the US by Chinese President Hu Jintao.
President Hu had been due to visit the US in September, but the visit was cancelled amid the aftermath of Hurricane Katrina.
"Clearly Hu Jintao is planning a trip to the US in the near future and the US government has probably made it clear that they want some kind of gestures on the human rights front," Dui Hua's research manager Joshua Rosenzweig told Reuters news agency.
High-level corruption
Mr Jiang was sentenced in June 2001 on charges of illegally supplying state secrets and inciting subversion after he wrote several articles about alleged corruption involving high-level government officials, including the provincial governor, Bo Xilai.
Mr Bo is now China's commerce minister.
Mr Jiang was originally given an eight-year sentence which would have run until January 2009. In March 2003, the Liaoning Intermediate People's Court reduced the sentence on appeal to six years - a sentence which would have expired in January 2007.
His imprisonment was met with an outcry from international rights groups and many governments, including the US, had appealed for his release.
The Dui Hua Foundation says Mr Jiang is with family members, but said it was likely he would apply for a passport in order to join his wife and daughter, who live in Canada.
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