Francisco Franco Suelves, 33, and his brother Juan José, 30, were named as signatories on two companies registered in the British Virgin Islands in the leaked Panama Papers seen by Spain's online newspaper El Confidencial.
The revelation that two of Franco's heirs held offshore companies came as little surprise in Spain where there have been calls for the family to return assets to the nation of Spain.
The family assets are broadly estimated at somewhere between €300 million and €400 million, thanks to properties and business interests accumulated during the 36-year dictatorship.
The Panama Papers scandal erupted on Sunday when media groups made public an investigation into a trove of 11.5 million documents leaked from Mossack Fonseca, a Panamanian law firm, on offshore accounts held by prominent individuals.
While offshore holdings can be legal, they can also be used to hide wealth in tax havens.
Among those involved are friends of Russian President Vladimir Putin, the brother-in-law of Chinese leader Xi Jinping, and Iceland's Prime Minister Sigmundur David Gunnlaugsson who has since resigned.
The papers have proved a headache for other prominent figures in Spain, including Pilar de Borbon, the aunt of Spain's King Felipe VI,as well as Barça star Lionel Messi and film director Pedro Almodovar.
Neither of the two great-grandsons of Franco wanted to comment on the revelations, although their father Francis Franco insisted: "It is all perfectly legal."
"It's not such a big thing to have companies in Panama," he told El Confidencial.