Mourad Laachraoui, brother of Najim Laachraoui, won the -54kg final in Montreux, Switzerland, on Thursday.
He beat 18-year-old Spanish fighter Jesus Tortosa 6-3 to claim the title.
The 21-year-old, who will compete at Rio 2016, has said he was "scared and saddened" by his brother's role in the attack on the Belgian capital in March.
Speaking at a news conference he arranged in Brussels following the attacks, Mourad Laachraoui also said no-one in his family had heard from his brother since 2013.
Najim, 24, was one of two men who carried out devastating bomb attacks in the check-in area of Zaventem airport on 22 March. A third bomb was detonated at Maelbeek metro station shortly after, with a total of 32 people killed.
Prosecutors have also linked him to November's attacks in Paris in which 130 people died.
A Belgian whose older brother was one of the suicide bombers in the terror attacks on Brussels has won gold at the European Taekwondo Championships.