The boy, who doesn't look older than 11 or 12, has allegedly just executed two men said to have confessed to spying for Russia, the voiceover saying their bodies "lay humiliated" at the feet of the young jihadist.
Their fate could not be independently confirmed.
When asked in another video what he wanted to do in the future, the boy said: "I will be the one who slaughters you, O kuffar (unbelievers). I will be a mujahed (holy warrior)."
The video is one of many examples of propaganda aimed at spreading IS's beliefs to a younger generation to aid recruitment and to perpetuate the "caliphate" the group has declared in areas it holds in Iraq and Syria.
Charles Lister, a visiting fellow at the Brookings Doha Center, said that over the past six months IS media materials "have steadily escalated the level of violence involving and being carried out by children, with this video being the most extreme level reached so far".
"Showing off children undertaking such violent acts is (IS's) way of suggesting its fight is one that should be undertaken by all men of supposed 'fighting age'," he said.
Also, indoctrinating young boys "into such a violent mindset helps to ensure the environment in which (IS) is now operating will be (one) in which it can continue to recruit from in the years to come."
One of the IS videos shows young boys undergoing classroom instruction in religion and Arabic, then firearms practice and physical training.
The jihadist group makes its aim clear.
"They are the next generation," a narrator says as one camouflage-clad boy quickly assembles a Kalashnikov assault rifle while others look on.
The boy, who calls himself Abdullah and says he is from Kazakhstan, is the same one who "executed" the two alleged spies.
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'DAMAGING TO CHILDREN'
Jeffrey Bates, the spokesman for the UN children's agency UNICEF in Iraq, said ideological indoctrination or participation in violence is incredibly damaging psychologically to the children who undergo it, and poses a major problem for the country.
"We have examples from around the world for decades of the impact this has on children, and it's devastating," Bates said.
"Not only are they using this as a tool to recruit people, but also to create a future where they have children who become adults who (move) this world view forward," he said of IS and other armed groups.
"The scope of the problem in Iraq -- you cannot overestimate it. We're looking at... thousands and thousands of children" who will need help.
Young boys feature in IS propaganda photos, including one showing a group of children wearing ski masks, dressed in camouflage uniforms and holding Kalashnikov rifles in front of an IS sign.
Another shows a very young child with two Kalashnikovs, wearing a magazine carrier and standing in men's combat boots that are far too big for him.
IS photos said to be from the jihadist stronghold of Raqa in Syria also show young girls wearing Muslim headscarves holding toy assault rifles.
Aymenn al-Tamimi, a fellow at the Middle East Forum, said the use of children in IS propaganda became more prevalent from mid-2013 onward, when its predecessor organisation "was trying to demonstrate its influence inside Syria".
IS is not the only militant group to use children for these purposes, but "for IS, it is a more distinct message because IS presents itself as an actual state," Tamimi said.
"The group uses children in its propaganda because it sees children as essential to perpetuate the Islamic state's existence into the next generation," he said.