Following a trial by the People's Court in Nghe An province that lasted half a day Tuesday, Hoang Duc Binh was convicted of abusing democratic freedoms to infringe on the interests of the state, organisation and people and opposing officers on duty, lawyer Ha Huy Son said.
Fellow activist Nguyen Nam Phong was given 2 years in jail for opposing officers on duty.
During last February's livestream on Facebook, Binh commented that the fishermen were stopped and beaten by authorities. Son said Binh told the court that he made the comments, but he denied committing a crime because what he said was true.
The US was "deeply troubled" by the two activists' convictions and sentences "under vague articles" of the law, State Department spokesman Heather Nauert said in a statement from Washington.
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The USD 10.6 billion steel complex owned by Formosa Plastics Group, which includes a steel plant, a power plant and a deep sea port in Ha Tinh province, discharged toxins such as cyanide and phenol during a test run in April 2016.
It killed massive amounts of fish and other sea life along more than 200 kilometers (125 miles) of coastline, devastating fishing communities and tourism in four central provinces. The plant owner has paid USD 500 million in compensation.
Despite economic reforms three decades ago that opened up the communist-ruled country to foreign investment and trade, making it one of fastest growing economies in the region, the one-party state maintains tight control on all aspects of life including the media and has zero tolerance for dissent.
Eight people have been convicted and given prison sentences for spreading propaganda against the state over the past month.
International human rights groups and some Western governments have criticised Vietnam for punishing those who peacefully express their views. Hanoi maintains that only law breakers are punished.