The Gulf Centre for Human Rights said Nadhir al-Majid, 40, received the sentence last week from the Specialised Criminal Court in Riyadh.
Rights monitors have criticised the practice of trying activists in such courts, which handle "terrorism" cases.
"Reports have confirmed that the writer was alone during the hearing and not accompanied by his family or his lawyer," said the Gulf Centre, which has offices in Copenhagen and Beirut.
Another watchdog, Human Rights Watch, in 2011 identified Majid as one of more than 160 dissidents arrested, mostly in Eastern Province where the Shiite minority had protested for political reforms and the release of prisoners.
He was freed in 2012, the Gulf Centre said.
In early January London-based Amnesty International said "a string of activists" had been detained or appeared in court over previous weeks in connection with peaceful human rights work.
"Saudi Arabia's authorities have begun the year with an intensified crackdown against human rights activists", it said.
Philip Alston, the UN special rapporteur on extreme poverty and human rights, said he received reports of "instances in which it has cracked down on certain people" communicating over the internet.
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