The court in Mallorca ordered the seizure of properties owned by Inaki Urdangarin, husband of the king's daughter Cristina, to cover a €6.1 million ($8.2 million) bond for his liability in the case, it said in a written ruling.
The court is investigating accusations that Urdangarin, an ex-Olympic handball player, and his former business partner Diego Torres embezzled €6 million in public funds.
The money was allegedly placed in the non-profit Nóos Institute, which Urdangarin chaired from 2004 to 2006 and of which Cristina was a board member, for it to organize sports events.
In a separate case, the judge investigating the allegations, Jose Castro, has also ordered the tax office to examine Cristina's financial affairs.
The three deny any wrongdoing and have not been formally charged with any crime. Cristina is not a formal suspect in the so-called Nóos case.
The scandal has nevertheless plunged Juan Carlos's family into its worst popularity crisis in his nearly four-decade reign, sharpening scrutiny of the royals as Spain suffers from five yeas of economic turmoil.
The list of homes, garages and other buildings detailed in Monday's ruling included Urdangarin's share of a luxury villa which he owns with Cristina in Barcelona.
It also included properties of companies owned by the couple and Torres and was aimed at covering the court bond imposed on both men.
Juan Carlos won respect for his role in Spain's transition to democracy after the death of the dictator Francisco Franco in 1975.
But the Nóos scandal, which erupted in late 2011, has since combined with his health problems to raise debate about the king's future.
In September he had his third operation in three years — a hip replacement for which another follow-up operation is scheduled on November 21st.
The king himself sparked outrage last year for taking an expensive elephant-hunting holiday in Botswana, while Spain struggled through a recession with one in four people out of work.
He broke his right hip during the trip and had to be flown home for surgery. Afterwards, he issued an unprecedented public apology for the hunting trip — seen as a sign of changing times for Spain's constitutional monarchy.