Acanthaster (from Ancient Greek ἄκανθα (ákantha), meaning "spine, thorn", and ἀστήρ (astḗr), meaning "star", and thus, "spine star/thorn star") is a bitypic genus of large and venomous starfish placed in the monotypic family, Acanthasteridae. Its two members are known as crown-of-thorns starfish. Acanthaster are native to coral reefs in Indo-Pacific region.

The species in this genus are a contributor to coral reef degradation because they prey on a large amount of live coral at high density.

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