Sonando, who is the leader of the campaign group Association of Democrats and owns Beehive radio, which broadcasts programmes critical of the government, was also fined 10 million riel (1,934 euros).
He was arrested in July, accused of involvement in a plot to establish an autonomous refion in eastern Kratie province.
Rights groups, including the Cambodian Centre for Human Rights and Amnesty International, say the charges are baseless and that the government is trying to justify a crackdown, in which soldiers shot dead a teenage girl during a demonstration over a land dispute in the province in May.
The European Union expressed “serious concern” about the verdict, saying that it “raises severe doubts about the impartiality and independence of the court”.
The French foreign ministry “deplored” it and called on the authorities to allow the question to be reexamined “in a fair and equitable manner”.
It also called on Cambodia to “respect freedom of expression and opinion”.
Three codefendants were jailed for between 10 months and five years.
Several hundred people demonstrated outside the court and Sonando’s wife, Din Phanara, said they would appeal.
Sonando was arrested in 2003 and 2005 for his political activities and defaming Prime Minister Hun Sen.
Environmental campaigners accuse the government of allowing vast swathes of forest to be destroyed. In 2011 concessions covering 800,000 hectares were given to companies close to the government, they claim.