The 54-year-old did not speak to reporters as he arrived for the court hearing in Pozuelo de Alarcon, an upscale Madrid suburb where he used to live, which comes just over 48 hours before he is set to return to Stamford Bridge to face old side Chelsea on Sunday.
The Portuguese coach, who wore a suit with no tie, is the latest high-profile football figure to be questioned over his tax affairs.
He entered the main door of the court without his lawyers, who arrived before him.
Spanish prosecutors accuse Mourinho, who coached Madrid between 2010 and 2013, of failing to declare income of 1.6 million euros in 2011 and 1.7 million euros in 2012.
The basis for the case, as with a series of football stars based in Spain, is how income from Mourinho's image rights was managed and declared.
Prosecutors believe by ceding his image rights to a series of companies based in tax havens, Mourinho committed fraud by not declaring the income those companies made from his image rights.
Mourinho is just one of a number of super agent Jorge Mendes's clients to be investigated in Spain, including World Player of the Year Cristiano Ronaldo.
Revelations from the whistleblowing website Football Leaks have lifted the veil on the practices supposedly used by Mendes to optimise often enormous earnings from image rights by his clients.
Media consortium European Investigative Collaborations(EIC) has claimed that no less than 185 million euros worth of income escaped the attention of tax authorities through the use of shell companies and offshore accounts.
Other clients of Mendes to be investigated in Spain include the former Real defenders Fabio Coentrao and Ricardo Carvalho, Colombian striker Radamel Falcao, now with Monaco but formerly at Atletico Madrid, and Paris Saint Germain's Angel di Maria, who also spent four years at Real Madrid.
Ronaldo, Messi tax woes
A statement in June from Mendes's Gestifute agency claimed Mourinho had already settled his tax obligation in Spain.
“Jose Mourinho, who lived in Spain from June 2010 until May 2013, paid more than 26 million euros in taxes, with an average tax rate over 41 percent, and accepted the regularisation proposals made by the Spanish tax authorities in 2015 regarding the years of 2011 and 2012 and entered into a settlement agreement regarding 2013.
“The Spanish Government in turn, through the Tax Department, issued a certificate in which it attested that he had regularised his position and was in compliance with all his tax obligations.”
In July, Ronaldo gave evidence at the same Pozuelo de Alarcon court in northwest Madrid as Mourinho, accused of evading 14.7 million euros through offshore companies between 2011 and 2014.
A judge is yet to decide whether Ronaldo's case will proceed to trial.
Barcelona's five-time World Player of the Year Lionel Messi was sentenced to a 21-month suspended jail sentence and 2.09 million euro fine last year for tax fraud related to his image rights.
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However, the jail term was later replaced by an extra 252,000 euro fine.
Messi and his father Jorge Horacio Messi were found guilty of using companies in Belize, the United Kingdom, Switzerland and Uruguay to avoid paying taxes on 4.16 million euros of Messi's income earned from his image rights from 2007 to 2009.
Barcelona defender Javier Mascherano also agreed a one-year suspended sentence with authorities for tax fraud last year.