American Airlines Flight 77 was a scheduled domestic transcontinental passenger flight from Dulles International Airport in Northern Virginia to Los Angeles International Airport in Los Angeles. 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 hijacked airliner was deliberately crashed into the Pentagon in Arlington County, Virginia, killing all 64 aboard and another 125 in the building.

Flight 77 became airborne at 08:20 ET. Thirty-one minutes after takeoff, the attackers stormed the cockpit and forced the passengers and crew to the rear of the cabin, threatening the hostages but initially sparing all of them. Lead hijacker Hani Hanjour assumed control of the aircraft after having undergone extensive flight training as part of his preparation for the attack. In the meantime, two people aboard discreetly made phone calls to family members and relayed information on the situation without the knowledge of their assailants.

Hanjour flew the airplane into the west side of the Pentagon at 09:37. Many people witnessed the impact, and news sources began reporting on the incident within minutes, but no clear footage of the crash itself is available to the public. The 757 severely damaged an area of the Pentagon and caused a large fire that took several days to extinguish. By 10:10, the damage inflicted by the plane and ignited jet fuel led to a localized collapse of the Pentagon's western flank, followed forty minutes later by another five stories of the structure. Flight 77 was the third of four passenger jets to be commandeered by terrorists that morning, and the last to reach a target intended by al-Qaeda. The hijacking was to be coordinated with that of United Airlines Flight 93, which was flown in the direction of Washington, D.C., the U.S. capital. The terrorists on Flight 93 had their sights set on a federal government building not far from the Pentagon, but were forced to crash the plane in a Pennsylvania field when the passengers fought for control after being alerted to the previous suicide attacks, including Flight 77's.

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.