Nhat was found guilty of “abusing democratic freedoms to infringe on the interests of the state” at a half-day trial in his native city of Danang on the central coast.
The broad anti-state charges can carry a maximum of seven years in jail.
“He strongly denied the charge, claiming that he is innocent,” said lawyer Tran Vu Hai.
The US said it was “deeply concerned” by the sentence.
“We call on the Vietnamese government to release Truong Duy Nhat and all prisoners of conscience, and allow all Vietnamese to peacefully express their political views,” an embassy statement said.
Nhat, 50, was a journalist working for several state-run papers before he quit and set up the popular blog “A Different Viewpoint”.
Private media are banned in Vietnam with all newspapers and television channels state-run, but many citizens now prefer to access news through blogs and social media.
Nhat’s posts frequently dealt with highly sensitive political issues and offered alternative commentary to the staid official press.
In a post in April 2013 Nhat called for Vietnam’s top leaders to resign. He said it was “time for a new party general secretary and prime minister” to save the nation from economic and political woes.
He was taken into police custody in May that year and his blog was shut down.
According to a copy of the indictment posted online, his articles “were not true (and) defamed leaders of the party and state, creating a one-sided pessimistic viewpoint”.
At the trial “Nhat said he should have been appreciated for being a good citizen as it is normal to criticise party and state leaders”, lawyer Hai told AFP.
AFP’s request to attend the trial was turned down by authorities.
On Monday New York-based Human Rights Watch called on the government to let Nhat walk free, saying he could not be jailed merely for disagreeing with the government and the party.
“Truong Duy Nhat’s trial is part of the Vietnamese government’s futile effort to silence the increasingly effervescent community of Vietnamese bloggers,” said Brad Adams, Asia director at Human Rights Watch.
In Vietnam, lawyers, bloggers and activists are regularly subject to arbitrary arrest and detention, according to rights groups.
Reporters Without Borders said Vietnam was second only to China in the number of bloggers it detained, with at least 34 currently behind bars.
In February the country was criticised at the UN Human Rights Council in Geneva during its periodic review over its treatment of regime critics.