Prosecutors had charged the man identified as Faycal C on Saturday with offences including "terrorist murder", and had been investigating the theory that he was a third airport attacker who fled when his bomb failed to go off.
His release comes as a new blow to an inquiry already dogged by accusations that Belgium missed a series of leads in cracking down on a jihadist network linked to the attacks and a similar assault in Paris in November.
Officials also updated the death toll from the bombings at the airport and at Maalbeek metro station to 35, the worst terror attacks in Belgium's history, after four more people had died in hospital.
Mourners were set to hold an Easter Monday church service in memory of the victims.
The Belgian federal prosecutor's office said in a statement that "the indications that led to the arrest of Faycal C were not substantiated by the ongoing inquiry. As a result, the subject has been released by the examining magistrate."
Belgian media had identified the man as Faycal Cheffou who claimed to be a freelance journalist.
With the manhunt still underway, police released video of a man in a hat and white jacket pushing a trolley with a large bag through the departure hall next to suicide bombers Ibrahim El Bakraoui and Najim Laachraoui.
As Belgium struggles to come to terms with the tragedy, recriminations continue over whether the authorities could have prevented it, as the links to the Paris attacks in which 130 people were killed become clearer by the day.
Metro bomber Khalid El Bakraoui, Ibrahim's brother, is meanwhile believed to have rented a property linked to Paris prime suspect Salah Abdeslam, who was arrested in Brussels on March 18 just yards from his family home after four months on the run.
And Turkey accused Belgium last week of ignoring a clear and present danger after revealing it had deported Ibrahim El Bakraoui as a "terrorist fighter" last year after arresting him near the Syrian border.
Two Belgian ministers offered to resign after the Turkish link emerged.