The toxic dump in 2016 by Taiwanese steel firm Formosa destroyed livelihoods along swathes of coast in central Vietnam and sparked rare nationwide demonstrations that saw several activists put behind bars.
Hoàng Đức Bình, 34, was convicted of “abusing democratic freedom” and “resisting public officers” following a half-day trial in the central province of Nghệ An held under heavy security, his lawyer told AFP.
“The court today had no grounds to give such a harsh sentence,” lawyer Ha Huy Son said.
Bình’s co-accused Nguyễn Nam Phong, 38, was given two years in prison for “resisting public officers” after he was stopped in a car along with Bình at an anti-Formosa protest in February 2017.
The court said he expressed remorse, while his co-accused Bình did not, according to Nghệ An’s official newspaper.
Bình, a well-known blogger and activist, was also accused of posting anti-government material on Facebook after the April 2016 fish kill.
At the time of his arrest in May 2017, he was the head of the activist group Vietnam Labour Movement and was accused of stoking unrest by “luring” fishermen in central Vietnam to join and organize protests, the Nghệ An newspaper reported.
The verdict was a “lesson and a warning to those intending to disrespect laws”, the official mouthpiece added.
Formosa, which was building an $11 billion (RM43 billion) steel firm at the time of the massive toxic leak, was ordered to pay $500 million (RM1.96 billion) to the Vietnamese government after the disaster.
But scores of fishermen said they were underpaid or received nothing at all, and tried to sue the government, which blocked their lawsuits.
Others joined regular protests which were broken up by security forces, sometimes violently.
Bình and Phong join scores of activists already behind bars in the one-party state, including at least eight jailed this year.
All independent media is barred in Vietnam, and activists have increasingly moved online to voice discontent.
Yet authorities also police the web and have jailed scores of bloggers for criticizing the regime.
A hard-line leadership in place since 2016 is accused of tightening its grip on activists, with at least 24 activists convicted last year and another 28 arrested, according to Human Rights Watch.