The trial stemmed from an incident in 2012 when the man was fighting in the Syrian civil for the anti-al-Assad forces there. His victim was fighting for the Assad regime.
The 28-year-old had since moved to Sweden and was granted a residency permit. The main evidence for the trial was a video which showed him and others beating a restrained captive.
Prosecutor Henrik Söderman speaking with Swedish Radio's reporter said that the court tried the man for war crimes because the beating was connected with an armed conflict.
"The deed itself, an extremely serious assault, was committed against a person who was in custody and in all likelihood detained because of the armed conflict in Syria. That was the basis for a war crimes trial," said Söderman.
Police became aware of the video when a friend of the perpetrator saw it and decided to report the crime. The 28-year-old had apparently shown the video on Facebook according to the court judgement.
After first denying his involvement, the man admitted to the beating though he allegedly told the police that he tried to "hit with care." The 28-year-old later claimed in court that he had been forced to beat the captive. But the court found that it was clear the man had acted of his own free will.
The Södertörns district court sentenced the man to five years in prison. His lawyer, Magnus Strömberg, told Dagens Nyheter that the man had previously indicated he would appeal a conviction.
Prosecutors had also pushed to have the man deported for life. While the court did not hand down that punishment, Dagens Nyheter reported that the Migration Board would consider revoking the man's residency permit.
Prosecutor Söderman indicated that the court considering a beating a war crime was new legal territory.
"From our perspective, it's interesting that the district court chose to issue a judgement for war crimes. As far as we can tell this is the first time this type of behavior has been found to constitute a war crime in Syria. That was something we were interested in trying," he said.
The 28-year-old is one of only three people who were found guilty of war crimes in Sweden during the last decade.