The investigation follows the leak of more than 11.5 million documents from the files of law firm Mossack Fonseca, based in the tax haven of Panama, revealing details of hundreds of thousands of clients, including in Spain.
The clients in Spain include the family of Barcelona football star Lionel Messi, an aunt of Spain's King Felipe VI, Pilar de Borbon, and Oscar-winning Spanish director Pedro Almodovar.
"We have opened an investigation for money laundering in relation to the law firm" Mossack Fonseca, a judicial source at the National Court, Spain´s top criminal court, told AFP.
The scandal erupted Sunday when media groups made public the results of a year-long worldwide investigation onto the trove of documents, which cover a period from 1977 until last December and expose a tangle of offshore financial dealings by the elite.
It comes at a time of mounting public concern in Spain over corruption scandals which have hit political parties of both the left and right, unions, footballers and even the royal family at a time of sky-high unemployment and government austerity measures.
While offshore holdings can be legal, they can also be used to hide wealth.
"Having an offshore holding is not a crime. What matters is knowing if these holdings in Panama, with Spanish shareholders or owners, are fullfilling their fiscal obligations," Spanish Justice Minister Rafael Catala said earlier on Monday when asked about the case.