The State Security Court on Sunday said Aktham Naisse, chairman of the Committees for the Defence of Democratic Liberties and Human Rights in Syria, was exonerated of all charges.
Syria's official news agency, SANA, said Naisse was acquitted because his case lacked "legal evidence", adding that some of the charges were dropped under a 2004 presidential amnesty.
Naisse was arrested in April 2004 on charges of spreading false information, forming an underground group, having links to international human rights organisations and other charges related to opposing the ruling Baath party.
He was released from prison four months later on $200 bail.
Naisse could not be reached for comment, but his lawyer, Anwar al-Bunni, said the State Security Court "was unconstitutional, illegal and unjust", despite Sunday's acquittal.
"(Four) months in prison, who can compensate for that?" he asked.
SANA said Sunday's hearing was attended by Naisse's lawyers and representatives of foreign diplomatic missions in Damascus.
Decisions by the State Security Court, which was created under Syria's 1963 emergency laws and where cases are often heard by military judges, cannot be appealed.
Ammar Qurabi, spokesman for the Arab Human Rights Organisation, called the verdict "excellent" and expressed hope that other political prisoners would be released soon.
Kurds say they face persecution
and harassment in Syria
The charges against Naisse are believed to stem from his human rights activities. In March 2004 he organised a rare protest outside Syria's parliament, which security agents quickly broke up. He and other activists were detained for a few hours and released.
Separately, the court on Sunday sentenced three Kurds to prison terms ranging from 30 to 36 months for "belonging to a banned political party".
There are about 1.5 million Kurds in Syria, a nation of 18.5 million, including about 160,000 who are denied Syrian citizenship. They often complain of harassment and persecution by security authorities.