Hoang Duc Binh, 34, was convicted of "abusing democratic freedom" and "resisting public officers" following a half-day trial in the central province of Nghe 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.
Binh's co-accused Nguyen Nam Phong, 38, was given two years in prison for "resisting public officers" after he was stopped in a car along with Binh at an anti-Formosa protest in February 2017.
The court said he expressed remorse, while his co-accused Binh did not, according to Nghe An's official newspaper.
Binh, 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 organise protests, the Nghe 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 steel firm at the time of the massive toxic leak, was ordered to pay $500 million 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.
Binh 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 criticising the regime.
A hardline 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.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)