The Syrian Human Rights League, or SHRL, said Muhannad al-Hassani, 44, was also charged with "weakening national sentiments" - a term often used in Syria against those who challenge the regime.
Al-Hassani headed the Syrian Organization for Human Rights - a separate group from the League. As a prominent lawyer, he handled cases of political prisoners in Syria and led calls for their release.
He was an outspoken defender of 12 jailed pro-democracy activists arrested two years ago. The group was part of a coalition known as the "Damascus Declaration" that called for peaceful democratic change in Syria.
Al-Hassani himself was arrested in July after being summoned several times for questioning by security agents.
Last month, 10 international rights groups awarded al-Hassani their annual prize for bravery, the Martin Ennals Award, saying he "bravely defended human rights."
The 20,000 Swiss francs ($18,000) prize is awarded jointly by groups including Amnesty International, Human Rights Watch and the International Federation for Human Rights.
A Syrian lawyer who follows activists' trials, Khalil Maatouk, described Wednesday's sentencing as "harsh and unjust" and called for al-Hassani's immediate release.
The SHRL also expressed grave concern over the sentencing, saying it met "none of the minimum conditions and criteria for a fair trail" and called for the immediate release of all political prisoners in Syria.
Arrests of pro-democracy activists are common in Syria, where several prominent political activists and writers remain imprisoned.