American Airlines Flight 77 was a scheduled American Airlines domestic transcontinental passenger flight from Washington Dulles International Airport in Dulles, Virginia, to Los Angeles International Airport in Los Angeles, California. The Boeing 757-223 aircraft serving the flight was hijacked by five al-Qaeda terrorists on the morning of September 11, 2001, as part of the September 11 attacks. The hijackers deliberately crashed the plane into the Pentagon in Arlington County, Virginia, near Washington, D.C., killing all 64 aboard (including six crew and themselves) and another 125 in the building.

31 minutes after takeoff, the hijackers stormed the cockpit and forced the passengers, crew, and pilots to the rear of the aircraft. Hani Hanjour, one of the hijackers who was trained as a pilot, assumed control of the flight. Unknown to the hijackers, passengers aboard made telephone calls to friends and family and relayed information on the hijacking.

The hijackers crashed the aircraft into the western side of the Pentagon at 09:37 AM ET. Many people witnessed the crash, and news sources began reporting on the incident within minutes. The impact severely damaged an area of the Pentagon and caused a large fire that took several days to extinguish. A portion of the building collapsed.

The damaged sections of the Pentagon were rebuilt in 2002, with occupants moving back into the completed areas that August. The 184 victims of the attack are memorialized in the Pentagon Memorial adjacent to the crash site. The 1.93-acre (7,800 m²) park contains a bench for each of the victims, arranged according to their year of birth, ranging from 1930 to 1998.