As a member of an armed Islamist group, 46-year-old Haisam Omar Sakhanh took part in a May 2012 attack to capture the seven men and in their execution two days later, the Stockholm district court said in a statement.
He killed one of the soldiers with an assault rifle, it said.
In early 2013, Sakhanh left Syria and sought asylum in Sweden, where he was granted a residence permit.
The New York Times had in September 2013 published a video showing him participating in the execution of the soldiers, who were not identified.
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The Stockholm court rejected his claim of a legitimate trial, and said his actions were "a violation of international humanitarian law" and "a serious crime against the law of nations."
"In this case it has been proven that less than two days passed between the capturing of the soldiers and the execution," the court said.
"The victims had substantial injuries after having been subjected to grievous bodily harm and entirely lacked the ability to defend themselves," the court said.
Before fighting in Syria, Sakhanh spent 10 years in Italy working as an electrician in Milan, according to Italian media.
Sakhanh was arrested in February 2012 in Rome for violent behaviour during a protest against President Bashar al-Assad outside the Syrian embassy in the Italian capital but did not attend his trial the following month.
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