The 1755 Lisbon earthquake, also known as the Great Lisbon earthquake, occurs in the Kingdom of Portugal on Saturday, 1 November, the holy day of All Saints' Day, at around 09:40 local time.

In combination with subsequent fires and a tsunami, the earthquake almost totally destroys Lisbon and adjoining areas.

Seismologists today estimate the Lisbon earthquake had a magnitude in the range 8.5–9.0 on the moment magnitude scale, with its epicenter in the Atlantic Ocean about two hundred kilometers (one hundred and twenty miles) west-southwest of Cape St. Vincent.

Estimates place the death toll in Lisbon alone between ten thousand and one hundred thousand people, making it one of the deadliest earthquakes in history.

The earthquake will accentuate political tensions in the Kingdom of Portugal and profoundly disrupt the country's colonial ambitions.

The event will be widely discussed and dwelt upon by European Enlightenment philosophers, and inspire major developments in theodicy.

As the first earthquake studied scientifically for its effects over a large area, it will lead to the birth of modern seismology and earthquake engineering.