Japan Air Lines Flight 123 (also known as JAL123) was a scheduled domestic Japan Air Lines passenger flight from Tokyo's Haneda Airport to Osaka International Airport, Japan. On August 12, 1985, a Boeing 747SR operating this route suffered a sudden decompression 12 minutes into the flight, and crashed in the area of Mount Takamagahara, Ueno, Gunma Prefecture, 100 km (62 mi; 54 nmi) from Tokyo 32 minutes later. The crash site was on Osutaka Ridge, near Mount Osutaka.

Japan's Aircraft Accident Investigation Commission (AAIC) concluded,: 129  agreeing with investigators from the U.S. National Transportation Safety Board, that the rapid decompression was caused by a faulty repair by Boeing technicians after a tailstrike incident during a landing at Osaka Airport in 1978 as JAL Flight 115. The rear bulkhead of the plane had been repaired with an improperly installed doubler plate, compromising the plane's airworthiness. Cabin pressurization continued to expand and contract the improperly repaired bulkhead until the day of the accident, when the faulty repair failed, causing a rapid decompression that ripped off a large portion of the tail and caused the loss of hydraulic controls to the entire plane.

The aircraft, which was configured with increased economy-class seating, was carrying 524 people. Casualties of the crash included all 15 crew members and 505 of the 509 passengers. Some of the passengers survived the initial crash but died of their injuries hours later while awaiting rescue. It is the deadliest single-aircraft accident in aviation history.