Nguyen Ngoc Nhu Quynh, who blogged under the name of Me Nam (Mother Mushroom), was put on a flight to the United States along with her mother and two young children.
Her release comes during a visit to Vietnam by the US Defence Secretary, Jim Mattis, and may be interpreted as a gesture of conciliation towards Washington.
However, the Communist authorities have increasingly opted for exile as a way to marginalise some of their most vocal critics. Some jailed dissidents believe they were subjected to harsh treatment to force them to agree to leave the country.
A number of other prominent government critics, including the human rights lawyer Nguyen Van Dai, have been sent into exile in western countries in recent years.
Quynh was sentenced to ten years in prison in 2017 on a charge of conducting propaganda against the state.
She was arrested in 2016 after joining a highly sensitive environmental campaign against the Taiwanese steel company, Formosa.
She had been subjected to harassment for years beforehand but escaped imprisonment despite her investigations of police brutality and other abuses of power.
Quynh complained of intolerable treatment and conditions in jail and went on a number of hunger strikes in protest. She was moved to a jail in the north of the country, hundreds of miles from her mother and children, in what appeared to be an attempt by the authorities to break her will.
Her cause was taken up by international rights groups and some western governments. She received the state department’s International Woman of Courage Award in 2017, presented to her in absentia by Melania Trump.
Some dissidents have refused offers of exile overseas for fear they will become less relevant in the struggle for political freedoms in Vietnam.
The regime’s growing intolerance of dissent, including the arrest of some 50 bloggers this year alone, has dashed hopes of a gradual liberalisation of the political system.