The man is suspected of involvement in the twin explosions which tore through Zaventem airport.
He was seen on CCTV pushing a trolley through the airport with two other suspects.
Both those men are believed to have died in the blasts, possibly by detonating suicide devices.
So-called Islamic State (IS) said it was behind both the attacks in the Belgian capital.
An online statement from the group said the locations were “carefully selected” and warned of worse to come for “Crusader states allied against the Islamic State”.
Belgian prosecutor Frederic Van Leeuw said police were seeking a man wearing a hat and light-coloured jacket, who was pictured with two other men at the airport.
He said searches were taking place “in several parts of the country”, adding that an explosive device containing nails, chemical products and an IS flag were found in an apartment in the Schaerbeek area of Brussels.
The prosecutor said both blasts at the airport were believed to have been carried out by suicide bombers.
The explosions happened in quick succession shortly after 08:00 local time (07:00 GMT), with some victims reportedly hit by the second blast as they tried to escape the first.
Another bomb was later found and destroyed in a controlled detonation at the scene.