"I have no plan to travel to Italy," Mottaki told reporters.
Western diplomats had seen the June 25-27 event as a rare chance for Group of Eight nations to sit down with regional powers such as Iran to discuss shared goals for Afghanistan and Pakistan.
Iran, like other neighbors of Afghanistan and Pakistan, was invited to attend discussions on June 26.
But tensions between the West and Tehran have risen since Iran's disputed June 12 election, as the Islamic Republic accuses Western powers of supporting street protests against the re-election of hardline President Mahmoud Ahmadinejad.
Italy, which hosts the meeting in the northern city of Trieste, said Tuesday it would seek consensus from G8 ministers on some form of condemnation of an Iranian crackdown on protesters and the expulsion of journalists.
(Reporting by Zahra Hosseinian and Hossein Jaseb; Writing by Fredrik Dahl; Editing by Louise Ireland)
