Brahim (left): "Close" to al-Qaeda's "financial brain"
Ahmed Brahim, 57, is also alleged to have been a leading planner of the bomb attacks on the American embassies in Kenya and Tanzania which killed more than 200 people in August 1998.
Mr Brahim was seized on Saturday from his home in Sant Joan Despi, near Barcelona, in a joint operation by police from several countries including the United States, France and Germany.
Interior Minister Mariano Rajoy said Mr Brahim had a "close relationship" with Mamdouh Mahmud Salim, whom he described as the "financial brain" of Osama Bin Laden's network.
Mr Salim, alleged to be a founding member of al-Qaeda, is in prison in the US, accused of conspiracy in the 1998 embassy bombings.
"This is a heavy blow to the finance apparatus of this terrorist network," Mr Rajoy said of the arrest, which followed a seven-month investigation.
Investigation
Mr Brahim's wife, Pirjo, protested his innocence.
"My husband has been living in Spain for 15 years and has nothing to do with al-Qaeda," she told reporters.
Police said Mr Brahim owned boat-trading and public relations firms.
He had recently sent money to some European countries and computer equipment to a number of Arab countries, including Saudi Arabia, they said.
Four French citizens who were at Mr Brahim's home when he was seized were being investigated but had not been arrested, Mr Rajoy said.
The capture brings to nine the total number of al-Qaeda suspects arrested in Spain since the 11 September attacks on the US.