Spanish media reports that Mourinho will be investigated over two alleged instances of tax fraud 2011 and 2012, when he was manager of Real Madrid.
Mourinho is alleged to owe €3.3 million, according to a statement from the prosecutor.
The revelation comes amid news Cristiano Ronaldo, who played under his Portuguese compatriot at Madrid, will testify at his own tax fraud trial in next month.
Ronaldo will appear in court on 31 July amid allegations of tax evasion.
Spanish media claimed the Real Madrid star must be in the courtroom in Pozuelo at 11am local time for the beginning of the case.
Ronaldo has been accused by prosecutors of defrauding Spanish tax authorities out of €14.7million, a claim he has strenuously denied.
The four-time Ballon d'Or winner is alleged to have created a "business structure" in 2010 to conceal his earnings from image rights between 2011 and 2014.
The allegations have led to uncertainty about the 32 year-old's future at the Santiago Bernabeu, with Manchester United linked as a possible destination should he opt to leave Spain.
Madrid president Florentino Perez defended the superstar on Monday and insisted he would not be letting his prized asset leave unless a club can meet his €1billion buyout clause.
Earlier this year Barcelona star Lionel Messi had a suspended 21-month jail sentence for tax evasion upheld by the Spanish Supreme Court.