United Airlines Flight 175 was a scheduled U.S. domestic passenger flight from Logan International Airport in Boston, Massachusetts to Los Angeles International Airport. The flight was hijacked by five al-Qaeda-associated Islamist terrorists, and flown into the South Tower of the World Trade Center in New York City as part of the September 11 attacks. Over thirty minutes into the flight, the hijackers forcefully breached the cockpit and overpowered the pilot and first officer. Marwan al-Shehhi, who was trained as a pilot, took over the controls. Air traffic controllers noticed the flight was in distress when the crew stopped responding to them. Several passengers and crew aboard made phone calls from the plane and provided information about the hijackers and injuries to passengers and crew.

The aircraft crashed into the South Tower of the World Trade Center at 09:03, killing all 65 people aboard, including the hijackers. The crash was seen on live television. The impact and subsequent fire caused the South Tower to collapse, which resulted in hundreds of additional casualties. During the recovery effort at the World Trade Center site, workers recovered and identified remains from Flight 175 victims, but many other body fragments could not be identified. The flight was the second plane hijacked and flown into the World Trade Center, but was the one airline crash that morning to be witnessed live on television around the world as it happened. It was preceded by American Airlines Flight 11, which had struck the top of the North Tower 17 minutes earlier.