The 2004 Indian Ocean earthquake was an undersea earthquake that occurred at 00:58:53 UTC December 26, 2004, with an epicentre off the west coast of Sumatra, Indonesia. The earthquake triggered a series of devastating tsunamis along the coasts of most landmasses bordering the Indian Ocean, killing more than 225,000 people in eleven countries, and inundating coastal communities with waves up to . It was one of the deadliest natural disasters in history. Indonesia, Sri Lanka, India, and Thailand were hardest hit.

With a magnitude of between 9.1 and 9.3, it is the second largest earthquake ever recorded on a seismograph. This earthquake had the longest duration of faulting ever observed, between 8.3 and 10 minutes. It caused the entire planet to vibrate as much as 1 cm (0.5 inches) and triggered other earthquakes as far away as Alaska.

The disaster is known by the scientific community as the Great Sumatra-Andaman earthquake, and is also known as the Asian Tsunami and the Boxing Day Tsunami. The tsunami occurred exactly one year after the 2003 Bam earthquake and exactly two years before the 2006 Hengchun earthquake.

The plight of the many affected people and countries prompted a widespread humanitarian response. In all, the worldwide community donated more than $7 billion (2004 U.S. dollars) in humanitarian aid.