Photos of his crucified body have been floating around the Internet, and apparently there was a message pinned to the corpse.
The message describes the ruling for the alleged crime as "apostasy" and states the teenager has been "killed and crucified for a period of three days" as the punishment.
The alleged execution comes after it emerged ISIS militants had beheaded their own fighters for spying and espionage.
It is not known who took the picture, which was circulated across social media by some ISIS supporters on Friday.
According to Charlie Winter of Quilliam Foundation- an anti-extremism think tank-
"Crucifixion has been used many times before - it's an age-old punishment dealt out to people who have committed treason."
He said this punishment arises from Isis's fundamentalist interpretation of Verse 33 of the fifth book of the Koran, which reads: "Indeed, the penalty for those who wage war against Allah and His Messenger and strive upon earth (to cause) corruption is none but that they be killed or crucified or that their hands and feet be cut off from opposite sides or that they be exiled from the land.
"That is for them a disgrace in this world; and for them in the Hereafter is a great punishment."
However, he said the next very next verse emphasizes forgiveness and removes the imperative to use such a punishment, saying: "Except for those who return (repenting) before you apprehend them. And know that Allah is Forgiving and Merciful."
Source: Times Of India