Civil Guard police said the searches were carried out in the southern coastal towns of Marbella and Puerto Banus with the aid of French police.
They followed a request by National Court judge Jose de la Mata who is probing money-laundering crimes carried out by a gang in the two towns, a court statement said.
Two of Rifaat Assad's wives and six of his sons are among the 15 people investigated.
Most of them are located in Puerto Banus, a luxury marina in Costa del Sol. The property stock, valued at 691 million euros (USD 735 million), includes a 33-square kilometer (12.7 sq mile) estate valued at 60 million euros.
The accounts of 76 "legal entities" which include companies, trusts and funds that were owned, administered or linked to Rifaat Assad and his relatives were also blocked, a court statement said.
The court said that no arrests were made.
Rifaat Assad is the exiled uncle of Syrian President Bashar Assad. He was vice president of Syria when the country was ruled by the current leader's father.
Rifaat was critical of Bashar after the country's crisis began in March 2011, vowing to work to topple the Syrian president and give the power to the people.
Spain opened the money laundering investigation last December following one started by France in 2013. Last June, France's financial prosecutor's office filed preliminary charges against Rifaat Assad for embezzlement and other counts.
In a 2011 interview, Rifaat Assad told The Associated Press that he had lost all his money in the stock market and lived off the largesse of his 16 children. His son Siwar said at the time that the holdings mostly included real estate, TV networks, hotels and a restaurant in Syria.
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