Specifically, he told world leaders on Thursday that the "majority of the American people, as well as most nations and politicians around the world agree" that "some segments within the US government orchestrated" the September 11, 2001, attacks in order to "reverse the declining American economy" and to justify US military operations in the Middle East to "save the Zionist regime".
Western diplomats, including the US, Canada and the 27-member EU bloc, walked out during the tirade. PJ Crowley, the US assistant secretary of state, told Al Jazeera that the statement was "totally outrageous".
But, for better or worse, significant segments of the world's population are sympathetic to Ahmadinejad's "conspiratorial" view of the 9/11 attacks which killed about 3,000 people.
The US government has determined that 19 hijackers, mostly born in Saudi Arabia and belonging to al-Qaeda, crashed two passenger jets into the World Trade Centre in New York City and one into the Pentagon, located outside Washington, DC, on September, 11, 2001.
A fourth plane crashed into a field in rural Pennsylvania after some of its passengers attempted to retake control of the aircraft. There were no survivors from any of the flights.
Some of the group's members, including Osama bin Laden, the head of al-Qaeda, had trained in Afghanistan prior to the attacks - training which precipitated the 2001 US-led invasion of the country.
Bin Laden initially denied, but later admitted in a taped statement aired on Al Jazeera in 2004, planning the attacks.
About 46 per cent of the world's people believe that al-Qaeda launched the 9/11 attacks, while 15 per cent think the US government was behind the assault, and seven per cent blame Israel, according to a
2008 world public opinion study carried out by the Program on International Policy (PIPA) Attitudes at the University of Maryland, which interviewed 16,063 people worldwide.
But Ahmadinejad views himself as a leader in the Arab and Muslim worlds. And, in these regions, surveys show significant sectors of the population believe that the US and Israel launched the 9/11 attacks to meet their own geopolitical goals.
In Jordan, 31 per cent of those polled by PIPA believe Israel was behind the attacks, while only 11 per cent blame it on al-Qaeda. Likewise, 43 per cent of Egyptians blame Israel, and 12 per cent
think the US was responsible, while only 16 per cent think al-Qaeda brought down the towers.
A 2006 poll from Scrippsnews says 36 per cent of Americans consider it "very likely" or "somewhat likely" that US government officials either allowed the attacks to be carried or launched the attacks
The most popular website on so-called alternative views on the September 11, 2001, attacks seems to be 911truth.org. The site acts as a clearing house for an array of various views and refuses to articulate an exact position on who it believes launched the attacks and why. Rather, it poses a series of questions, while offering readers the "Top 40 Reasons to Doubt the Official Story".
A "documentary" called Loose Change questioning the official 9/11 narrative, has been popular among activist groups and other more traditional doubters.
"That 19 hijackers are going to completely bypass security and crash four commercial airliners in a span of two hours, with no interruption from the military forces, in the most guarded airspace in the United
States and the world? That to me is a conspiracy theory," Korey Rowe, the film's director, told Time magazine.
Mainstream media in the West, and even the US government itself, have felt a need to respond to these theories. The US state department's Bureau of International Information Programs has a website devoted to debunking the "top September 11 conspiracy theories".
In a piece titled "Why the 9/11 Conspiracy Theories Won't Go Away", Time argues that what many would call conspiracy theories are "not a fringe phenomenon".
Some of the more unconventional claims include that the twin towers did not collapse from the impact of two Boeing 767s and the petrol stored in their tanks. Rather, the buildings collapsed in a planned, controlled demolition - perhaps through explosions in the basement.
Another popular theory is that a missile fired by elements from within the US government hit the Pentagon, rather than an aircraft. Some of the "alternative" or "conspiratorial" views come from people
who seem like serious scholars.
The Centre for Research on Globalisation, a site which frequently publishes articles critical of the "deliberate suppression" of answers on 9/11 and the "falsification of evidence" by the US government, has Dr Michel Chossudovsky, a professor at the University of Ottawa and author of the well-researched book The Globalisation of Poverty, as one of its leading members.
Posting on Twitter, a user called Martynnorris says: "Which is more childish, the rantings of Ahmadinejad or the faux shock and the offended face?"
Regardless of what polite opinion thinks inside the Washington, DC, beltway, Ahmadinejad is speaking to a significant global constituency. There is little evidence to suggest that they include "the majority of the American people, as well as most nations and politicians around the world", as the Iranian leader said in his UN speech. But the 9/11 "conspiracy theories" are not a fringe phenomenon either.