Twenty-six-year-old Suliman al-S. was arrested in January 2016, becoming the first asylum seeker in Germany to face charges over alleged war crimes committed in Syria.
The court in the southwestern city of Stuttgart found that while the suspect did not take part in the 2013 abduction of a Canadian UN observer, he aided and abetted the kidnappers by acting as a guard.
The peacekeeper with the UN Disengagement Observer Force (UNDOF), who was stationed in the Golan Heights but abducted in Damascus, managed to free himself after eight months in captivity.
Prosecutors had sought a seven-year sentence for Suliman al-S., who arrived in Germany in 2014.
The judges said they had not seen conclusive proof that the suspect belonged to a branch of the Al-Nusra Front jihadist group, as alleged by the prosecution.
The peacekeeper's identity was never revealed by German prosecutors, but the UN said in 2013 that Carl Campeau, a Canadian legal advisor, was abducted on February 17 as he drove through a Damascus suburb.
He was freed in October, without a ransom being paid, according to the UN.
German federal prosecutors have opened over a dozen investigations concerning alleged war crimes in Syria or Iraq, alongside dozens of cases of suspected membership in jihadist groups.
The investigations have gained momentum with the arrival of more than one million asylum seekers since 2015, including hundreds of thousands from Syria and Iraq.
In July 2016, in the first such conviction, a German jihadist was sentenced to two years in prison on war crimes charges after posing for pictures in Syria with the severed and impaled heads of two government soldiers.