An amphitheatre (British English) or amphitheater (American English; both ) is an open-air venue used for entertainment, performances, and sports. The term derives from the ancient Greek ἀμφιθέατρον (amphitheatron), from ἀμφί (amphi), meaning "on both sides" or "around" and θέατρον (théātron), meaning "place for viewing".

Ancient Roman amphitheatres were oval or circular in plan, with seating tiers that surrounded the central performance area, like a modern open-air stadium. In contrast, both ancient Greek and ancient Roman theatres were built in a semicircle, with tiered seating rising on one side of the performance area.

Modern parlance uses "amphitheatre" for any structure with sloping seating, including theatre-style stages with spectator seating on only one side, theatres in the round, and stadia. They can be indoor or outdoor.

Natural formations of similar shape are sometimes known as natural amphitheatres.