The official said Thursday Hussein Salem, one of the most secretive businessmen in Egypt, was detained in Majorca on an international arrest warrant issued by Interpol.
Salem was charged last month along with Mubarak and the ex-president's two sons. Their trial is scheduled for August.
The Egyptian protesters who forced Mubarak from office in February are pushing for prosecution of Mubarak and his cronies for what they say were years of abuse and corruption.
Salem, 77, is said to have won lucrative land and other deals, including exporting gas to Israel, because of his connection to Mubarak.