The reporter, Ahmad Rafat, says he was forced to surrender his accreditation to the World Food Security Summit, which is being held at the UN Food and Agriculture Organization's (FAO) headquarters.
The Iranian-born Rafat is based in Rome and is a regular correspondent for Radio Farda, a joint venture between RFE/RL and Voice of America that broadcasts to Iran.
He said he was told by Italian guards that he had been declared persona non grata and that a security officer had "made it understood that this was because of pressure [by] a foreign delegation."
"I think my reports about the protests of political [groups] over Ahmadinejad's trip to Italy and the fact that no political force or official is willing to meet with him -- the reports which I did for Radio Farda in the past days and also for the news agency [Adnkronos] for which I am the editor in chief here in Italy -- are the only issues that could have caused this reaction," Rafat told Radio Farda. "I haven't done anything but write."
Rafat has been an active advocate of media and human rights, including criticism in the days before the FAO event of Tehran's rights record.
He has reported extensively for Radio Farda on preparations for Iranian President Mahmud Ahmadinejad's visit to Rome and his unsuccessful efforts to arrange a meeting with Pope Benedict XVI.
In an appeal to the event's organizers, RFE/RL President Jeffrey Gedmin said the broadcaster was "disappointed at the designation of any professional, accredited journalist anywhere as 'persona non grata,'" and he said the company was "particularly concerned that this could be possible at a UN-sponsored conference convened in Italy."
European Parliament Vice President Mario Mauro criticized the snub as the "gravest" of mistakes that risks giving the impression that an undemocratic country is forcing its will upon the international community.
Ahmadinejad has accused "big powers" of manipulating food supplies to achieve "political and economic aims." A strident critic of the United States, Ahmadinejad argued at the FAO summit against tackling the food crisis within the auspices of the United Nations, which he has accused of being a "tool" of Washington and other major powers.