The blogger known as Mother Mushroom has been sentenced to 10 years in prison on a charge of conducting propaganda against the state.
The sentence is one of the harshest for a political activist and human rights campaigner in recent years and was imposed following a rapid one day trial.
The blogger, whose real name is Nguyen Ngoc Nhu Quynh, denied the charges against her and insisted she had been exercising her right to freedom of expression.
Her lawyer said the sentence was too harsh and unjust and that his client intended to appeal.
Quynh’s supporters and independent reporters were denied access to the court room.
State media said she was accused by prosecutors of using Facebook to edit, post, and share articles with fake content, with the intention of propagating distorted propaganda against the state.
It said she had lashed out against the policies of the Communist party and the Socialist Republic of Vietnam, distorting the history of the revolution and undermining national unity.
Prosecutors had requested a sentence of between 8 and 10 years.
Witnesses said that Quynh had been rebuffed when requested a delay in the trial to prepare her case with her lawyers.
Her mother, who was allowed to watch on a screen from a neighbouring room, said her daughter was constantly interrupted when she tried to make her case.
State media said prosecutors also alleged that she had illegally received money from a civil society organisation overseas.
Human rights groups condemned the charges against her as outrageous, saying she was being persecuted for exercising her right to free expression to hold the government to account.
Amnesty International said Vietnam should stop criminalising people for peacefully expressing the right to freedom of expression and should respect the right to a fair trial.”
Quynh was well known for her commentaries on police violence, abuse of power and environmental issues before her arrest last October.
She was denied all access to visitors until her mother was allowed to see her for five minutes on the eve of the trial.
“The scandal here is not what Mother Mushroom said,” wrote Phil Robertson, deputy Asia director at Human Rights Watch.
“(It is) Hanoi’s stubborn refusal to repeal draconian, rights-abusing laws that punish peaceful dissent and tarnish Vietnam’s international reputation.”
Quynh was arrested on October 10, the day after she attempted to visit a fellow activist in prison.
She had been subject to police threats and harassment for years and had been detained for short periods on a number of occasions, often to prevent her meeting visiting foreign delegations.
Her commentaries on last year’s devastating chemical spill off the north-central coast are thought to have antagonised the government and could have led to the decision to prosecute her.
The authorities have since arrested a number of activists involved in the environmental campaign as they attempt to crush a protest movement involving victims of the environmental disaster.
The United States honoured Quynh with an “International Woman of Courage” award in March this year in a clear expression of support for the jailed blogger.
Other western governments and human rights organisations have also called for her immediate release.