IS killed the 19-year-old male student on Thursday after kidnapping more than 30 people, mostly women and children, from a village in Sweida during a deadly rampage last week, the head of the Sweida24 news website Nour Radwan told AFP.
Quoting relatives, Radwan said the young man was taken from the village of Al-Shabki on July 25 along with his mother.
His family received two videos, the first showing him being decapitated and the second of him speaking before being killed as well as images of his body after his death, Radwan said.
Sweida24 posted online part of a second video, which was seen by AFP, showing a young man who appeared to be sitting on the ground in a rocky landscape.
His is wearing a black T-shirt and his hands are tied behind his back.
The video could not be independently verified.
The Syrian Observatory for Human Rights, a Britain-based war monitor, said it was the first execution since the kidnappings.
On July 25, IS carried out a series of attacks in Sweida's provincial capital and several villages that killed more than 250 people, mostly civilians.
It was the deadliest attack ever to target the mostly government-held province and the secretive Druze religious minority that populates it.
During the attack the jihadists abducted 36 Druze women and children from a village in Sweida's east, the Observatory said at the time.
Four women had since escaped while two had died, leaving 14 women and 16 children in IS captivity, according to the Observatory.
At the time, another 17 men were unaccounted for but it was unclear if they were also kidnapped.
On Friday, a top Druze religious leader said regime ally Russia was in talks with the jihadists over their release.
Sweida had until last week largely remained isolated from Syria's seven-year conflict.
Druze, which made up three percent of Syria's population before 2011, are considered Muslim but IS see them as heretics.