IS went on a rampage in Sweida on July 25, killing more than 250 people - mostly civilians - in the deadliest attack ever to target the mostly government-held province and its Druze religious minority.
The jihadists also kidnapped more than 30 people, most of them women and children, from a village in the province, which had previously remained largely isolated from Syria's seven-year civil war.
On Thursday, IS killed a 19-year-old male student who was among the hostages, the head of the Sweida24 news website Nour Radwan told AFP.
Quoting relatives, Radwan, who was speaking from Sweida, 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 bearded young man who appeared to be sitting on the ground in a landscape of grey rocks.
He is wearing a black T-shirt and tracksuit bottoms, and his hands are tied behind his back.
IS has not claimed the kidnappings and did not publish the video on its usual channels.
IS jihadists have lost much of the territory they once controlled in Syria after overrunning large swathes of it in 2014, but they retain a presence in the east of the country and in the vast Badiya desert that sweeps through its south.