Vietnamese dissident Nguyen Ngoc Nhu Quynh, known as "Mother Mushroom", was released from prison on Wednesday and had left Vietnam on a flight to the United States, sources with direct knowledge of the matter said.
Quynh, one of Vietnam's most prominent dissidents, was serving a 10-year-sentence for anti-state propaganda.
"Mother Mushroom departed Vietnam today for the US," one source told Reuters.
"She is flying with her children and mother."
Vietnam's foreign ministry did not immediately respond to a request for comment.
The US Embassy in Vietnam referred Reuters questions to the Vietnamese government. Quynh's mother was not reachable by telephone.
News of the release came shortly after US Defense Secretary Jim Mattis left Vietnam. It was not known if the release had any connection his visit. Mattis arrived there on Tuesday and left for Singaporeon Wednesday.
A Pentagon spokesman did not immediately comment when asked about the timing of the release.
Despite sweeping economic reform in Vietnam, and increasing openness towards social change, Vietnam's ruling Communist Party does not tolerate criticism that it deems threatens its rule.
Quynh, a blogger and environmental activist, was arrested for posting what police described as anti-state reports, including one about civilians dying in police custody.
She was jailed for 10 years last year for publishing propaganda against the state, following an increase in a crackdown on dissidents since 2016, when more conservative leaders cemented their power in top party positions.
Quynh is the second dissident released this year.
A prominent human rights lawyer, Nguyen Van Dai, was released from prison in June and went to Germany.
A screening of a documentary featuring Mother Mushroom, When Mother's Away, was cancelled on July 4 after a complaint from the Vietnamese embassy in Bangkok.
The complaint was followed by a police visit to the Foreign Correspondents Club of Thailand (FCCT), where was the film was to be screened for the second time.