Judge Jose Castro of Palma said there is evidence the princess may have known her husband was using her name and status to advance his questionable business deals, The New York Times reported. Castro said he was swayed, in part, by emails from a former associate of Cristina's husband, Inaki Urdangarin, a former Olympic handball player.
The princess is to appear in court April 27.
Urdangarin is suspected of using his royal status to secure inflated sports contracts from regional politicians for his foundation, Instituto Noos, and then directing millions of euros to other companies and offshore accounts controlled by him and his associates, the newspaper said.
Cristina is the first royal-born person to be called before court in modern Spanish history, the Times said.
Urdangarin has denied any wrongdoing, and has not been charged with any crime.
A spokesman for the royal household said Wednesday it was "surprised" by the subpoena because the judge had indicated a year ago there was not enough evidence to name the princess as a suspect and the main prosecutor had lodged an appeal against the judge's ruling.
Benedict Cumberbatch's dramatic reading of R. Kelly lyrics is just what you need
Gal Gadot cast as Wonder Woman for 'Batman vs. Superman'