Haisam Sakhanh, a Syrian with a Swedish residency permit, was arrested in Örebro in March 2016 over suspicions that he had shot a person with an assault rifle during the execution carried out by Islamist armed group the Suleiman Company (Firqat Suleiman el-Muqatila).
During interrogation he confirmed that he was involved in the shooting, but denied breaching international law.
In 2013 the New York Times published a graphic video of the incident showing the execution of the seven men who had their hands tied behind their backs. They were shot dead from a close range, with shots fired at their heads and bodies.
The clip was used as evidence in the Stockholm district court trial of Sakhanh, who sought asylum in Sweden in 2013 and has lived there since.
The defence argued that the execution was carried out by order, and was the enforcement of a death sentence by a court following a trial, something which became a key issue in the trial.
"A big question in the judicial process has been whether a non-governmental actor can establish their own courts to maintain law and order within the framework of a non-international armed conflict," presiding judge Tomas Zander explained in a press release.
The court concluded that may be possible under certain circumstances, but in this case it was proven that less than two days passed between the capturing of the soldiers and the execution, which was one of the reasons it was decided that the execution was not preceded by a fair trial where a legitimate court sentenced the soldiers to death.
The court also judged that the connection between the execution and the non-international armed conflict in Syria meant the actions "entail serious violations of the regulations of international humanitarian law".
"The Defendant is therefore sentenced for serious crime against the law of nations," it concluded.
The verdict was hailed as an "important signal" by the head of the war crimes investigation group at the Swedish police.
"The judges have given an important signal that Sweden is not a safe haven for war criminals," Per Ahsltröm said in a statement.