"Today I discovered I was the cause of Italy's press freedom problems," wrote Grillo in an ironic blog post titled "It's all Beppe Grillo's fault".
In the annual World Press Freedom Index, released on Wednesday, the France-based group had said Italy had a "noticeable problem" with regards to freedom of the press.
Despite soaring 25 places in the global rankings, Italy was one of the lowest-scoring countries in Western Europe, due to intimidation from organized crime groups as well as pressure from politicians which led journalists "increasingly to censor themselves", Reporters Without Borders wrote.
The organization singled out Grillo, who is notoriously critical of Italy's mainstream press, for "publicly outing" journalists he disagreed with on his blog.
"It opened my eyes," said Grillo. "It's all my fault. I thought that [Italy's lack of press freedom] was due to the political parties who have put their own men in the management of [national broadcaster] Rai and the TV news programmes, telling them what to say and what not to say."
Grillo noted that former prime minister Silvio Berlusconi, who leads the Forza Italia party, is a media tycoon who remains the controlling shareholder of Italy's largest commercial broadcaster, Mediaset.
Meanwhile, Carlo de Benedetti, who owns the Espresso group which publishes the country's second most widely read newspaper, La Repubblica, has strong links to Italy's centre-left Democratic Party.
Grillo also hit back at the group's claims that he had "publicly outed" journalists he disliked, by listing his 2016 blog posts in which he had spoken about specific journalists, justifying each of them by saying he was calling out censorship or unverified reports.
He had also criticized one journalist for calling Gianluca Casaleggio, the Five Star Movement's co-founder, a "cretin" shortly after Casaleggio's death.
"Dear Reporters Without Borders," said Grillo. "Denouncing a fact (the objective non-existence of press freeom) and asking for a retraction of fake news cannot be considered intimidation."
He added that WikiLeaks founder Julian Assange had praised the party's efforts to "remove the veil of filters of the old, corrupt, mainstream media".
Grillo, a former comedian, has long accused Italy's mainstream media of creating fake news in order to damage his party. He has banned Five Star members from appearing on talk shows and gives international media priority over their Italian counterparts at his rallies, preferring to communicate via his blog.
In January, he called for the establishment of "public juries" to decide whether news was real or fake. One Democratic Party senator said the idea amounted to "fascism", while journalists' union FNSI slammed the "lynching of all journalists".
Grillo himself has been accused of fake news links, after a BuzzFeed News investigation in November uncovered connections between supposedly independent news sites and the party's leadership.