A man known as Faycal C, the only person arrested and charged with involvement in the Brussels attacks, has been released for lack of evidence.
Belgian media said the man had been suspected of being the mystery third man in CCTV footage of the bombers.
But a judge found there was no evidence to justify holding him, the prosecutor's office said.
In a statement, the Belgian federal prosecutor's office said: "The clues that led to the arrest of Faycal C were not substantiated by the ongoing inquiry.
He had been charged only two days before with "taking part in a terrorist group, terrorist murder and attempted terrorist murder".
Belgian media say the charges remain, even though he has been released.
Belgian public TV and Le Soir daily identified the freed man as Faycal Cheffou, a freelance journalist.
Last Tuesday's attacks on the airport and the city's metro system killed 35 people and injured more than 300.
The attacks were claimed by the Islamic State (IS) militant group.