Officers, acting on a request from Madrid, swooped on the man's home in the city's Zavantem district, Belgian prosecutors said.
The Zavantem airport was targeted by a deadly Isis-claimed double suicide bombing last year.
The 26-year-old Spaniard, who has not been named, is suspected of leading a network of "recruitment and indoctrination from his home and in Brussels cafes", Spanish police said in a statement.
Unable to travel to Iraq or Syria with his brother to join the ranks of Isis fighters, the suspect planned "terrorist acts in Europe, particularly in Belgium", Spanish police said, adding that "specific targets" had been identified, including locations, buildings and groups of people.
He is also accused of showing other members of the cell "videos of suicide bombings, executions, death sentences and calls for terrorist attacks in Europe," the statement said.
Belgian police held the man under a European arrest warrant issued by a judge in Spain who has been investigating the cell for several years.
Four other members of the cell are already behind bars, three in Spain and one in Morocco.
The Belgian federal prosecutor said the suspect would be extradited to Spain, as he was not currently wanted by the authorities in Belgium.
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