Spanish newspaper El Mundo report that 55-year-old admitted two charges of tax fraud - brought to him by Spanish state prosecutors - which centred around undeclared image rights during his time at Real Madrid between 2011 and 2012.
Prosecution sources say the agreement has been 'sealed', while Mourinho commented that the matter is now 'closed'. The prosecutors are now expected to drop their case against the Manchester United boss, with a prison sentence once a possibility.
It was reported that Mourinho allegedly gave false information to Spain's Tax Agency during a 2015 investigation and, having already paid a six-figure fine, saw his case reopened.
He appeared in court again last November, where it was revealed that Mourinho had paid all of his taxes but that the Portuguese boss was still being investigated.
Mourinho was brought to court after his affairs were revealed by the European Investigative Collaborations Consortium, which includes Spanish newspaper El Mundo, Germany's Der Spiegel newspaper and the Sunday Times of Britain.
It is claimed that Mourinho was also benefiting from an offshore shell company, which the Spanish tax authorities were lead to believe was an active business.
Fellow Portuguese football star Cristiano Ronaldo is currently facing an ongoing tax fraud investigation. In a similar situation to Mourinho, Ronaldo allegedly evaded £13.1m on image rights between 2011 and 2014.
Mourinho and Ronaldo are both clients of well-renowned super-agent Jorge Mendes, who has previously been at the centre of a European crackdown on tax evasion cases.