Spain has ordered properties to be seized and bank accounts frozen in a money laundering investigation linked to the family of Rifaat al-Assad.
Judiciary officials said that the uncle of Syria’s President Bashar al-Assad was handed $300m of public money when he was sent into exile in 1984.
He was placed under formal investigation in France last year.
Some of his fortune ended up in property in Marbella and Puerto Banus on the south coast, officials said.
While his brother Hafez al-Assad was in power, Rifaat was renowned for the brutality with which he crushed an uprising in the Syrian city of Hama in 1982. Last year he was described as living in Paris.
He is under investigation in France for concealing stolen assets, money laundering and fiscal fraud. Several luxury properties have already been seized by French authorities in Paris.
Rifaat al-Assad has in the past said he was given the money by the Saudi royal family.