Pham Viet Dao, 61, a Communist Party member and a former official at the Ministry of Culture, was charged with "abusing democratic freedoms to infringe on the interests of the State" after a half-day trial in Hanoi.
In addition to his own writing, Dao's popular blog included dozens of links to other commentaries on politics and society.
His blog attracted thousands of hits between February 2012 and May 2013.
The court ruled that the posts "defamed the party and state... blackening the honour and prestige of the (communist) party leader and the prime minister," presiding judge Ngo Tu Hoc said.
"His online distortion and defaming of party and state leaders caused confusion over the party leadership and undermined people's trust in the party," Hoc added.
Dao, who appeared calm in court, apologised for "having posted some incorrect information," but added that he did not think his posts had "badly influenced society."
Ahead of the sentencing, New York-based Human Rights Watch on Tuesday called on the Vietnamese government to drop all charges against Dao and free him "immediately and unconditionally".
"The Vietnamese authorities are shaming themselves before domestic and international public opinion by staging yet another political trial of a peaceful critic," said Brad Adams, HRW's Asia director.
International rights groups said lawyers, bloggers and activists in the country are regularly subject to arbitrary arrest and detention.
Two weeks ago, popular blogger and journalist Truong Duy Nhat was jailed for two years for the same charge, which prompted the US to say it was "deeply concerned" by the sentence.
According to Reporters Without Borders, Vietnam was second only to China in the number of bloggers it detained, with at least 34 currently behind bars.
Communist Vietnam bans private media, with all its newspapers and television channels state-run. However, many citizens now prefer to access news through blogs and social media.