The raids, in the Puerto Banus marina area of the plush resort of Marbella, are the result of a corruption investigation launched in France against Assad's uncle, Rifaat al-Assad.
The assets of Rifaat al-Assad and his family in Spain are valued at 691 million euros ($736 million), the source said, adding that "all these properties have been seized."
"Judge Jose de la Mata is coordinating the judicial operation against Rifaat al-Assad, uncle of the current president of Syria, for money laundering in Puerto Banus and Marbella," the source said.
In 1984, Rifaat al-Assad was forced into exile for trying to overthrow his older brother, the late Syrian dictator Hafez al-Assad.
He had received 300 million dollars of state cash which he used to buy real estate in France, Liechtenstein, Luxembourg and Curacao, according to Spanish justice.
French and Spanish authorities have been cooperating on the case and Spain opened an investigation on the back of the one in France, where authorities suspect Rifaat al-Assad of misuse of Syrian public funds and money laundering.
In a statement, Rifaat al-Assad, now aged 79, and his family retorted that they had "never benefited from financing that in any way wronged the Syrian state and its people."
"They question the timing of these coordinated judicial actions, 33 years later, at a time when their country is suffering more than ever and where Rifaat al-Assad's voice should be heard to contribute to find a peaceful solution to the Syrian conflict," it read.
One residence, "La Maquina" at Benahavis just outside Marbella is worth an estimated 60 million euros.
The investigating magistrate has frozen 16 people's bank accounts and a further 76 accounts in connection with the case, the judicial source said.
First charged in France last June, Rifaat al-Assad holds some 90 million euros worth of property assets in France, including an office block in the east-central city of Lyon, investigators say.
He has previously said that much of his wealth stems from gifts bestowed by the Saudi royal family, which he says gave him political backing in the 1980s.
Last October, he told French authorities that former Saudi kings Fahd and Abdallah had provided those gifts, according to a source close to the French probe.
The initial probe was triggered by an activist group, which represents victims of financial crime.
Rifaat al-Assad insists he does not keep close tabs on his property holdings.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)