Younes Abaaoud, a 13 year-old from Belgium, is believed to have joined his older brother to fight in Syria for Isis.
The youngster's identity was tweeted by Shiraz Maher, an academic and senior fellow at the International Centre for the Study of Radicalisation (ICSR) at King's College, London.
The academic wrote: "One of the youngest foreign fighters we know of Younes Abaaoud, who was 13 when he left Belgium to join Isis."
The photo of Abaaoud shows him holding an AK-47 in his left hand and pointing to the sky with his right forefinger.
The boy's father, believed to be a shopkeeper called Omar, originally from Morocco, said the boy was taken to Syria after his brother Abdelhamid had been brainwashed in Belgium.
Abaaoud followed his older brother to the front line and is understood to be among dozens of child fighters in the war zone from Europe.
However, Belgium media reports could not clarify whether the 13 year-old is fighting or living away from the front lines.
Britain has among the highest number of extremists fighting for Isis – at least 500 – but Belgium has the highest proportion based on its population.
Research by the ICSR in London said that overall Western Europeans now represent up to one-in-five of the foreign fighter population in Syria.
European Isis fighters
It says the most come from France. The French government says that at least seven of those are children and 50 are women.
France is followed by Britain, Germany, Belgium and the Netherlands.
Adjusting for population size, the most heavily affected countries are Belgium – up to 27 foreign fighters per million, followed by Denmark, the Netherlands, Sweden, Norway and Austria.
The United Nations have voiced their condemnation of Isis for recruiting children.
"Armed opposition groups have been responsible for the recruitment and use of children both in combat and support roles, as well as for conducting military operations, including using terror tactics, in civilian-populated areas, leading to civilian casualties, including children," stated a United Nations report on children in Syria.