Terceira (Portuguese pronunciation: [tɨɾˈsɐjɾɐ]) is a volcanic island in the Azores archipelago, about a third of the way across the North Atlantic Ocean at a similar latitude to Portugal's capital Lisbon, with the island group forming an insular part of Portugal. It is one of the larger islands of the archipelago, with a population of 53,311 inhabitants in an area of approximately 396.75 km² (153.19 sq mi).

Terceira is the location of the Azores' oldest city, Angra do Heroísmo, the historical capital of the archipelago and UNESCO World Heritage Site; the seat of the judicial system (Supreme Court); and the main insular Portuguese Air Force base, Base Aérea nº 4 at Lajes, with a United States Air Force detachment.

Terceira island has two main sea ports, one at Angra do Heroísmo and the other at Praia da Vitória, and a commercial airport integrated with the flight operations at Base Aérea nº 4. The Portuguese bullfight is popular on the island, coming in two variations: the traditional equestrian bullfight (in the ring) and the popular "touradas à corda" that occur in the streets.