That's according to El Confidencial (h/t AS), who report the state are preparing a claim ahead of June 30, when the case is set to expire. The claim would involve returns between 2009 and 2011, when he is said to have diverted some of his earnings to the British Virgin Islands instead of reporting all of them.
Ronaldo has denied any wrongdoing.
It's not the first time the Portugal international has been linked with allegations of tax evasion. Agence-France Presse (via Yahoo Sports) reported a number of European outlets made similar claims during the Football Leaks releases in December.
At the time, the club released a statement saying the Spanish tax authorities absolved the striker.
A spokesman for the agency that represents Ronaldo, Gestifute, told El Confidencial (via AS) they believe no laws were broken: "In accordance with the "Beckham Act," which [Ronaldo] was subject to at the time in question, he was only obliged to pay tax on image rights earnings generated in Spain and not on those from abroad."
The spokesman also admitted the tax agency "maintains different criteria," however.