The case has drawn criticism from human rights groups and Western governments, including the United States.
Ammar Qorabi, head of the National Organisation of Human Rights in Syria, said the 12, all signatories of a Damascus Declaration calling for democratic change, were sentenced in a Damascus court.
Author Ali Abdullah, Dr Walid Bunni, writer Akram Bunni, and former MP Fidaa Horani were also convicted as were Ahmad Tomeh, Jabr al-Shufi, Yasser al-Iti, Mohammed Haji Darwish, Marwan al-Ish, Fayez Sara and Talal Abu-Dan.
George Bush, the US president, had called for Seif's release in a speech in July.
The sentences were likely to put more strain on Syrian-US ties which were damaged this week by a US military raid on the Syrian side of the border with Iraq.
Syria says the raid killed eight civilians while a US official said it was believed to have killed a smuggler of foreign fighters into Iraq.