Mohammed Ghanem was found guilty of "offending the president of the republic, damaging the image of the state and incitement to confessional dissent,” by a court in the town of Raqqa, the Syrian National Human Rights Organisation said.
The group denounced the sentence, saying it came "in the context of repressing human rights activists and others’ opinions that has been followed up by the Syrian authorities" for many months.
The military court initially sentenced Ghanem to a year in prison, but then reduced the term to six months.
Ghanem was arrested at his home on March 31 after he published articles online critical of the domestic situation in Syria.
Syria has detained more than 10 writers and human rights activists in recent months.