The Chi-Chi earthquake (later also known as the Jiji earthquake) (Chinese: 集集地震; pinyin: Jíjí dìzhèn; Wade–Giles: Chi2-Chi2 Ti4-chên4), also known as the great earthquake of September 21 (九二一大地震; Jiǔ-èr-yī dàdìzhèn; '921 earthquake'), was a 7.3 ML or 7.7 Mw earthquake which occurred in Jiji (Chi-Chi), Nantou County, Taiwan on Tuesday, 21 September 1999 at 01:47:12 local time. 2,415 people were killed, 11,305 injured, and NT$300 billion worth of damage was done. It is the second-deadliest earthquake in Taiwan's recorded history, after the 1935 Shinchiku-Taichū earthquake.

Rescue groups from around the world joined local relief workers and the Taiwanese military in digging out survivors, clearing rubble, restoring essential services and distributing food and other aid to the more than 100,000 people made homeless by the quake. The disaster, dubbed the "Quake of the Century" by the local media, had a profound effect on the economy of the island and the consciousness of the people, and dissatisfaction with government's performance in reacting to it was said by some commentators to be a factor in the unseating of the ruling Kuomintang party in the 2000 presidential election.