The September 11 attacks were condemned by world leaders and other political and religious representatives and the international media, as well as numerous memorials and services all over the world. The attacks were widely condemned by world governments, including those traditionally considered hostile to the United States, such as Cuba, Iran, Syria, Libya, North Korea, and Afghanistan. Reports of Arabs in New Jersey celebrating the attacks are uncorroborated and many have been linked to unsubstantiated conspiracy theories.

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 perpetrators of the attacks.

In the immediate aftermath of the attacks, the Bush administration declared a war on terror, with the stated goals of bringing Osama bin Laden and al-Qaeda to justice and preventing the emergence of other terrorist networks. These goals were to be accomplished by means including economic and military sanctions against states perceived as harboring terrorists and increasing global surveillance and intelligence sharing. Within hours after the September 11 attacks, Defense Secretary Rumsfeld speculated on possible involvement by Saddam Hussein and ordered his aides to make plans for striking Iraq; although unfounded, the association contributed to public acceptance for the 2003 invasion of Iraq. The second-biggest operation of the war on terror outside of the United States, and the largest directly connected to terrorism, was the overthrow of the Taliban's rule in Afghanistan by a US-led coalition.

President George W. Bush quoted Psalm 23 in a nationally-televised address immediately following the attacks:

America and our friends and allies join with all those who want peace and security in the world, and we stand together to win the war against terrorism. Tonight, I ask for your prayers for all those who grieve, for the children whose worlds have been shattered, for all whose sense of safety and security has been threatened. And I pray they will be comforted by a power greater than any of us, spoken through the ages in Psalm 23: "Even though I walk through the valley of the shadow of death, I fear no evil, for You are with me."

Bush also called for solidarity with Muslims and Islam following attacks on Arab-Americans and Muslims, stating "The face of terror is not the true faith of Islam. That's not what Islam is all about. Islam is peace. These terrorists don't represent peace. They represent evil and war."

In a joint statement by the American Muslim Alliance, American Muslim Council, Association of Muslim Scientists and Engineers, Association of Muslim Social Scientists, Council on American-Islamic Relations, Islamic Medical Association of North America, Islamic Circle of North America, Islamic Society of North America, Ministry of Imam W. Deen Mohammed, Muslim American Society and Muslim Public Affairs Council, stated:

American Muslims utterly condemn the vicious and cowardly acts of terrorism against innocent civilians. We join with all Americans in calling for the swift apprehension and punishment of the perpetrators. No political cause could ever be assisted by such immoral acts.