Nguyen Ngoc Anh, a 39-year old shrimp farmer from the southern town of Binh Dai, was arrested and tried for what state media described as "making, storing, releasing, and circulating information and documents against the state."
He was sentenced to six years in prison Thursday by the People's Court of Ben Tre province, followed by five years under house arrest, according to the state-run Vietnam News Agency (VNA).
The European Commission called the sentence a "direct breach" of international human rights agreements.
Vietnam's government has not responded to CNN's request for comment. The Southeast Asian country has been intensifying its efforts to stifle online speech.
A new cybersecurity law came into effect in January. Described by activists as "Stalinist," the law criminalizes criticism of the government and forces internet companies to store data locally and hand over user information to authorities without the need for a warrant.
A number of opposition bloggers have been imprisoned.
According to Human Rights Watch (HRW), Nguyen took part in environmental protests against Formosa Plastics Group, a Taiwanese steel company that admitted to being responsible for an environmental disaster in 2016. The company allegedly dumped toxic waste into the sea, killing huge numbers of marine creatures.
He also publicly boycotted the national election in May 2016 and repeatedly voiced support for Vietnam's political prisoners, said HRW.
"Through his images, video clips and documents, Anh fabricated information distorting and defaming authorities, raising public doubt and worry," VNA quoted the court as saying.
The European Commission said Nguyen's arrest was "a worrying development."
"The European Union expects the Vietnamese authorities to immediately release Mr. Nguyen Ngoc Anh as well as all bloggers and human right defenders imprisoned for having peacefully expressed their views," the spokesperson added.
The EU is set to sign a trade deal with Vietnam in the coming weeks and HRW said the bloc should use its power to demand human rights protection.
"Holding this trial just as the European Council is in the process of ratifying the EU-Vietnam Free Trade Agreement shows how ruthless the government can be and demonstrates why human rights improvements need to be part of trade deals and not sidelined in the name of diplomacy," the group said earlier this week.