Mendes has been called to give evidence along with two of Ronaldo's other advisors, Luis Correia and Carlos Osorio, on October 19th in the upmarket Pozuelo de Alarcon suburb of Madrid, home to many of the city's football stars.
Ronaldo strongly denied having evaded €14.7 million ($17.5 million) in tax on his image rights when he appeared in court in a preliminary hearing on July 31st.
Prosecutors accuse the Real Madrid star of evading tax via a shell company based in the British Virgin Islands and another in Ireland, known for low corporate tax rates.
In addition, they say Ronaldo only declared €11.5 million of Spanish-related income from 2011 to 2014, while what he really earned during that time was close to €43 million.
They also accuse him of “voluntarily” refusing to include €28.4 million in income linked to the sale of his image rights for the 2015 to 2020 period to a Spanish company.
According to Forbes, Ronaldo is the highest paid sports star in the world with a $93 million (€83 million) income between salary and endorsements in 2016-2017.
Mendes, one of football's leading agents, has already appeared in court to give evidence in a similar case against another of his clients, Radamel Falcao.
In June, Mendes told the court he “never advised players in tax matters”, and denied helping to create shell companies for his clients.
Monaco's Falcao is suspected of failing to correctly declare €5.6 million of income earned from image rights between 2012 and 2013 while he was at Atletico Madrid.
Mendes, a former nightclub manager, found himself in the eye of the storm last year in the “Football Leaks” media investigation into alleged corruption in the game.
According to newspaper El Mundo, who have led the Football Leaks investigation in Spain, Ronaldo also cleared Mendes of any wrong-doing in his testimony before the court.
Correia is in charge of promoting Ronaldo's image to potential sponsors, whilst Osorio is the player's lawyer.
Ronaldo is just the latest of a series of players facing tax investigations in Spain, many of them Mendes clients.
Among those in Mendes's stable, Manchester United manager Jose Mourinho, Paris Saint-Germain winger Angel di Maria and Portugal international defender Fabio Coentrao have been accused of tax fraud.
Barcelona's Lionel Messi was given a suspended 21-month jail sentence and fined two million euros for tax fraud worth €4.1 million in 2016.
His sentence, though, has since been replaced by an extra €252,000 fine.
If Ronaldo faces trial and is found guilty, he would risk “a fine of at least €28 million” and could potentially be jailed for three and a half years, according to the Gestha union of experts at Spain's Inland Revenue.