Jailed Islamist Fouad Belkacem, whose group Sharia4Belgium sent dozens of jihadists to Syria, has been stripped of his Belgian citizenship and faces deportation to Morocco.
The appeal court in Antwerp ruled that he had fallen seriously short of his duties as a citizen.
Belkacem was jailed in 2015 for leading a terror group, many of whose recruits joined jihadist group Islamic State.
More Belgians per capita went to fight in Syria than from any other EU state.
Some of those who returned to Europe were involved in the Paris attacks in 2015 and the Brussels bombings of March 2016.