The investigation relates to earnings from his image rights in 2013.
The 29-year-old left-back has already paid tax at the corporate rate but investigators suspect it may be liable at the personal rate.
Miles Dean, partner at Milestone International Tax, says the investigation is harder to justify a second time.
Hard done by
“Emboldened Spanish officials simply see footballers as rich pickings,” said tax expert Dean. “It appears Marcelo has been hard done by.”
“He settled his tax debts in 2013, which he might legitimately have expected to be the end of the story, only for the Spanish authorities to reopen the case on new grounds, this time targeting his personal tax affairs, rather than corporate.”
According to Dean, the Marcelo case shows that footballers are now seen as a soft target for investigators in Spain.
Marcelo, Mourinho and Ronaldo all deny any wrongdoing.