Le Dinh Luong, 53, was sentenced to twenty years in prison on a charge of plotting to overthrow the government, a charge often used to silence government critics in the one-party state.
The sentence was even longer than the 17 years formally requested by prosecutors.
Other activists suspect the communist authorities are trying to intimidate them following their success in organising a protest movement in north-central coastal provinces after a toxic leak from the Formosa steel plant in 2016.
Luong is not very well known nationally, but he became a prominent figure in the movement of Catholic groups and fishermen that demanded action over the Formosa scandal.
The government has tried a range of tactics: breaking up demonstrations and arresting organisers, as well as disciplining local officials and forcing Formosa to accept responsibility, in an attempt to defuse a protest movement that seriously challenged party authority in central provinces.
Luong’s defiance in court and refusal to plead guilty may have angered the authorities and persuaded them to increase the sentence beyond that previously agreed with prosecutors.
Vietnam’s courts make little attempt to display any degree of independence during such political cases. The hearings tend to last only a few hours, defence lawyers are given short shrift, and the result is almost guaranteed to be a prison term of varying length.
State media described Luong as a member of the US based opposition party, Viet Tan, which campaigns for democracy in Vietnam.
His lawyer said the trial was not fair and he would appeal.