The federal prosecutor's office did not confirm the arrest.
De Standaard newspaper said the man, whom it did not name, was arrested after a policeman recognised him from the security footage.
Khalid El Bakraoui, 27, who blew himself up on a crowded rush hour train, is the brother of Brahim El Bakraoui, who detonated a bomb at Brussels Airport an hour earlier.
The joint attacks killed at least 31 people and wounded 316.
Public broadcaster RTBF meanwhile reported police detained a seventh person in the district of Forest following six arrests late on Thursday.
- Reuters