The decision to strip all six men of their Belgian nationality followed their conviction in absentia for their links to the group, during a 2015 trial in which 44 people were convicted for terror-related activities.
A former student from Deurne, Antwerp, the family Akrich, or Abu Sayfurahmaan, said he was radicalised by the now-dissolved terror group before fleeing home for Syria in 2012.
Like all six men stripped of their nationality, Akrich could not be notified of the decision since his whereabouts were until now unknown.
Following the ruling by the court, Akrich and the other men have a window of eight days to file an appeal against the decision.
Akrich is said to have joined the ranks of the Syrian branch of Al Qaeda, Jabhat al-Nusra, and has been identified by Jejoen Bontinck, another departee to Syria, as one of them who tortured him while he was imprisoned there.
It is not clear how he ended up in Turkey, where many detainees with links to the Islamic State (IS) and other terror cells in the region fled following the chaos that erupted after a Turkish-led military operation in the north-east of Syria.
Akrich is one of several people with links to Belgium to have been arrested in Turkey, in particular women and children, but also two men, including one from Brussels who remains detained in the contry and Mehdi Aïda, who has recently been sentenced after his repatriation in 2017.