The 1755 Lisbon earthquake, also known as the Great Lisbon earthquake, impacted Portugal, the Iberian Peninsula, and Northwest Africa on the morning of Saturday, 1 November, Feast of All Saints, at around 09:40 local time. In combination with subsequent fires and a tsunami, the earthquake almost completely destroyed Lisbon and adjoining areas. Seismologists estimate the Lisbon earthquake had a magnitude of 7.7 or greater on the moment magnitude scale, with its epicenter in the Atlantic Ocean about 200 km (120 mi) west-southwest of Cape St. Vincent and about 290 km (180 mi) southwest of Lisbon.

Chronologically, it was the third known large scale earthquake to hit the city (following those of 1321 and 1531). Estimates place the death toll in Lisbon at between 12,000 and 50,000 people, making it one of the deadliest earthquakes in history.

The earthquake accentuated political tensions in Portugal and profoundly disrupted the country's colonial ambitions. The event was widely discussed and dwelt upon by European Enlightenment philosophers, and inspired major developments in theodicy. As the first earthquake studied scientifically for its effects over a large area, it led to the birth of modern seismology and earthquake engineering.