Omar al-Saeed is the fourth member of the Saudi Civil and Political Rights Association (ACPRA) to be jailed this year after the group issued statements attacking the ruling family over its human rights record and calling for democracy.
Saeed did not have legal representation at the secret hearing when he was sentenced, ACPRA said in a statement on its website.
"It’s just another troubling instance of Saudi authorities’ absolute refusal to countenance any activism or criticism of Saudi policies or human rights abuses," said Adam Coogle, Middle East researcher at Human Rights Watch.
A spokesman for the Justice Ministry said he could not comment on the report or confirm its accuracy.
U.S.-allied Saudi Arabia is ruled by the al-Saud family together with powerful clerics from the country’s ultra-conservative Wahhabi school of Sunni Islam.
King Abdullah is the prime minister and he appoints the government, which includes top princes in the roles of foreign, interior, defence and intelligence ministers. Most senior princes also have extensive business interests.
Saudi Arabia’s only elections are for half the seats on relatively powerless town councils. Licensed media exercises stringent self censorship, public expressions of dissent are often not tolerated and demonstrations are banned.
During the Arab Spring in early 2011, protests were restricted to the Shi’ite minority in the Qatif district of Eastern Province.
Unrest among the Sunni majority was avoided after King Abdullah pledged $110 billion in spending on social benefits and top clerics and tribal leaders said people should back the ruling family.
Foreign analysts say there appears to be little public demand for big political changes in Saudi Arabia now, although they point to evidence on social media of growing frustration at corruption, poverty and poor state services.
The government has denied charges by international rights groups that it used a campaign against Islamist militants over the past decade to stifle dissent.
However, human rights lawyers inside the country have said some of those sentenced in security courts, including a group jailed in Jeddah in late 2011, were peaceful activists put on trial for demanding political change.