A court in Jeddah handed down the sentence on Raif Badawi, who started the 'Free Social Liberals' website to discuss the role of religion in Saudi Arabia, Saudi media reported on Tuesday.
"This incredibly harsh sentence for a peaceful blogger makes a mockery of Saudi Arabia's claims that it supports reform and religious dialogue," said Nadim Houry, deputy Middle East director at Human Rights Watch.
The world's top oil exporter follows the strict Wahhabi school of extremism and judges base their decisions on their own interpretation of religious law instead of precedent or a written legal code.
Despite pushes for reforms to the legal system, Saudi lawyers say conservatives in the judiciary have resisted implementing many of those changes.
Badawi, 35, was arrested in June last year in the Red Sea city of Jeddah for unknown reasons.
The network that he co-founded with female rights activist Suad al-Shammari, had announced May 7, 2012 a "day of liberalism" in the Muslim kingdom.
More than 40,000 political prisoners, mostly prisoners of conscience, are in jails across Saudi Arabia.
According to the activists, most of the detained political thinkers are being held by the government without trial or legitimate charges and have been arrested for merely looking suspicious.
NJF/NJF