The Reactions to the September 11 attacks included condemnation from world leaders, other political and religious representatives and the international media, as well as numerous memorials and services all over the world. The attacks were denounced by the governments of countries traditionally considered hostile to the United States, such as Cuba, Iran, Libya and North Korea. However in some cases celebrations of the attacks were also reported, and some groups and individuals accused the United States in effect of bringing the attacks on itself.

In the immediate aftermath of the attacks support for the United States' right to defend itself was expressed across the world, and by United Nations Security Council Resolution 1368. Many countries introduced "anti-terrorism" legislation and froze the bank accounts of businesses and individuals they suspected of having connections with al-Qaeda and its leader Osama Bin-Laden, the presumed perpetrators of the attacks.