"We swear to God we will eat your hearts and livers, you soldiers of Bashar the dog," the insurgent says as he bends over a blurred body and cuts out the heart in the amateur footage uploaded to YouTube.
"We are the heroes of Baba Amr," he says, referring to a rebel stronghold of the central city of Homs that has been flattened by President Bashar al-Assad's forces.
The man, identified in the video as a member of the mainstream rebel Free Syrian Army, then stands up, raises his dagger in one hand and the heart in the other.
He then proceeds to place the organ in his mouth, before the video abruptly ends.
The authenticity of the footage, which was uploaded to YouTube on Sunday by a user named "SyrianGirl War", cannot be verified.
The Syria conflict flared when forces loyal to Assad launched a brutal crackdown on Arab Spring-inspired democracy protests that erupted in March 2011.
The Syrian Observatory for Human Rights, a Britain-based watchdog, says more than 80,000 people have since been killed, with numerous incidents of mass killings and other atrocities.