Nguyen Quoc Quan had been held in a Vietnamese prison for subversion charges related to his ties with the banned political party and democracy group Viet Tan, the Los Angeles Times reported Wednesday.
Vietnam has labeled Viet Tan a terrorist group though the United Nations human rights office and other watchdogs say it is a peaceful organization.
The official Vietnam News Agency said Nguyen was released because he pleaded guilty and "begged for clemency so he could return to the United States and reunite with his family."
Nguyen's wife, Huong Mai Ngo, and his lawyer, Do Hoang, said the confession never happened.
Do said he believes Nguyen was released because of "intense pressure from the international community" after another recent trial in which 14 activists were found guilty of the same crime and sentenced to 14 years in prison, the Times reported.