Two vehicles carrying four armed men drove up to a security van near the Swiss passenger plane, officials said, Belgian state broadcaster VRT reported.
"The men were armed and masked. There were no shots fired and nobody was injured," a spokeswoman for the Brussels prosecutor said.
Both vehicles sped off after the robbery and one of them was later found, said officials, who would not comment on what was taken.
The robbers managed to get onto the tarmac after breaking through the fence that surrounds the airport, a spokesman for Brussels Airport said.