The September 11 attacks (often referred to as 9/11) were a series of four coordinated terrorist attacks by the Islamist terrorist group al-Qaeda against the United States on the morning of Tuesday, September 11, 2001. The attacks resulted in 2,977 fatalities, over 25,000 injuries, and substantial long-term health consequences, in addition to at least $10 billion in infrastructure and property damage. 9/11 is the single deadliest terrorist attack in human history and the single deadliest incident for firefighters and law enforcement officers in the history of the United States, with 343 and 72 killed, respectively.

Four passenger airliners which had departed from airports in the northeastern United States bound for California were hijacked by 19 al-Qaeda terrorists. Two of the planes, American Airlines Flight 11 and United Airlines Flight 175, crashed into the North and South towers, respectively, of the World Trade Center complex in Lower Manhattan. Within an hour and 42 minutes, both 110-story towers collapsed. Debris and the resulting fires caused a partial or complete collapse of all other buildings in the World Trade Center complex, including the 47-story 7 World Trade Center tower, as well as significant damage to ten other large surrounding structures. A third plane, American Airlines Flight 77, was crashed into the Pentagon (the headquarters of the U.S. Department of Defense) in Arlington County, Virginia, which led to a partial collapse of the building's west side. The fourth plane, United Airlines Flight 93, was initially flown toward Washington, D.C., but crashed into a field in Stonycreek Township, Pennsylvania, after passengers thwarted the hijackers.

Suspicion quickly fell onto al-Qaeda. The United States responded by launching the War on Terror and invading Afghanistan to depose the Taliban, which had failed to comply with U.S. demands to expel al-Qaeda from Afghanistan and extradite their leader Osama bin Laden. Many countries strengthened their anti-terrorism legislation and expanded the powers of law enforcement and intelligence agencies to prevent terrorist attacks. Although bin Laden initially denied any involvement, in 2004 he claimed responsibility for the attacks. Al-Qaeda and bin Laden cited U.S. support of Israel, the presence of U.S. troops in Saudi Arabia, and sanctions against Iraq as motives. After evading capture for almost a decade, bin Laden was located in Pakistan in 2011 and killed during a U.S. military raid.

The destruction of the World Trade Center and nearby infrastructure seriously harmed the economy of New York City and had a significant effect on global markets. The U.S. and Canadian civilian airspaces were closed until September 13, while Wall Street was closed until September 17. Many closings, evacuations, and cancellations followed, out of respect or fear of further attacks. Cleanup of the World Trade Center site was completed in May 2002, and the Pentagon was repaired within a year. The construction of One World Trade Center began in November 2006, and the building opened in November 2014. Numerous memorials have been constructed, including the National September 11 Memorial & Museum in New York City, the Pentagon Memorial in Arlington County, Virginia, and the Flight 93 National Memorial at the Pennsylvania crash site.