United Airlines Flight 175 was a domestic passenger flight from Logan International Airport in Boston to Los Angeles International Airport in California that was hijacked by five al-Qaeda terrorists on the morning of September 11, 2001, as part of the September 11 attacks. The aircraft involved, a Boeing 767-200 carrying 65 passengers and crew, was deliberately crashed into the South Tower of the World Trade Center in New York City, killing everyone aboard and causing the deaths of more than 600 people in the South Tower's upper levels in addition to an unknown number of civilians and emergency personnel on floors beneath the impact zone. Flight 175's hijacking not only led to it being the second-deadliest of the four suicide attacks carried out on the day in terms of plane and ground fatalities, but also secured its place as second-deadliest plane crash in aviation history, surpassed only by American Airlines Flight 11.

The September 11 attacks, commonly known as 9/11, were four coordinated Islamist suicide terrorist attacks carried out by al-Qaeda against the United States on September 11, 2001. That morning, 19 terrorists hijacked four commercial airliners scheduled to travel from the East Coast to California. The hijackers crashed the first two planes into the Twin Towers of the World Trade Center in New York City, two of the world's five tallest buildings at the time, and aimed the next two flights toward targets in or near Washington, D.C., in an attack on the nation's capital. The third team succeeded in striking the Pentagon, the headquarters of the U.S. Department of Defense in Arlington County, Virginia, while the fourth plane crashed in rural Pennsylvania during a passenger revolt. The September 11 attacks killed 2,977 people, making them the deadliest terrorist attack in history, and instigated the multi-decade global war on terror, fought in Afghanistan, Iraq, and elsewhere.