Hassan al-Mandlawi, 32, and Al-Amin Sultan, 30, both Swedish nationals, were convicted after graphic videos showed them taking part in the killing of two men in the northern city of Aleppo which has been ravaged by the more than four-year war in Syria.
The footage, discovered on a USB stick in Sultan's home, shows two men speaking in Swedish and giving directions before one prisoner is beheaded and another has his throat slashed with a knife.
Both are heard cheering while one severed head is held up.
Investigators have not been able to identify the victims, who the court said were killed in a "very cruel and brutal manner".
The defendants, who were in court for the hearing in their hometown, Sweden's second city of Gothenburg, had denied they were involved in the executions.
But the court convicted Mandlawi and Sultan of "terror crimes" after the National Forensic Centre confirmed they were the men seen in the film and the court ruled they clearly played a commanding role in the killings.
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The two men, who are being held in custody, have said they will appeal.
Prosecutor Agnetha Hilding Qvarnstrom said they were actively involved in the murders, without actually holding the knife.
The court found that the murders and the videos "intended to instill fear" in the people of Syria and other countries who do not follow the killers' brand of Islam, according to a copy of the verdict.
Sultan denied it was his voice in the video and charged that the prosecution had come to the wrong conclusion because there were similarities between his clothing and those worn by the man seen in the footage.
Pictures of Sultan posing with weapons were also found on the USB stick, along with several videos of Mandlawi, who said he went to Syria to "support the people against the regime (of President Bashar al-Assad)".
Mandlawi is wheelchair-bound due to injuries suffered after being shot in the head. He claims to have been shot in Turkey, where he said he worked as a volunteer doing relief work although the prosecutor says he was shot while in Syria.
The southwestern city of Gothenburg has seen about 120 men leave the country to fight with the Islamic State group, according to police.
Monday's verdict came after neighbouring Finland on Friday remanded in custody Iraqi twin brothers suspected of killing 11 people in a massacre by the Islamic State group in Iraq in 2014.