Nguyen Huu Vinh, more commonly known as Anh Ba Sam, was arrested in 2014 and accused of disseminating anti-government articles on his wildly popular website.
Vinh, a 60-year-old former policeman, was convicted in March on charges of "abusing democratic freedoms" along with his assistant Thi Minh Thuy who was given three years in jail. The crime carries a maximum penalty of seven years.
"This is a totally imposed verdict, it isn't based on any proof," said his lawyer Ha Huy Son after the day-long appeal hearing.
Vinh, who appeared in court alongside Thuy, was sentenced under article 258 of the criminal code, which rights groups have condemned for being vaguely worded and used to silence dissidents.
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Vinh's site Ba Sam, which means "talking nonsense" in Vietnamese, attracted 3.7 million page views before it was taken down shortly after his arrest.
He founded the website in 2007, initially to store articles for his own reference, but the site later became a news aggregator with links to stories from state-run newspapers and also posts from activists.