Ocean acidification poses a range of threats to marine invertebrates; however, the potential effects of rising carbon dioxide (CO ₂ ) on marine invertebrate behaviour are largely unknown. Marine gastropod conch snails have a modified foot and operculum allowing them to leap backwards rapidly when faced with a predator, such as a venomous cone shell. Here, we show that projected near-future seawater CO ₂ levels (961 µatm) impair this escape behaviour during a predator–prey interaction. Elevated-CO ₂ halved the number of snails that jumped from the predator, increased their latency to jump and altered their escape trajectory. Physical ability to jump was not affected by elevated-CO ₂ indicating instead that decision-making was impaired. Antipredator behaviour was fully restored by treatment with gabazine, a GABA antagonist of some invertebrate nervous systems, indicating potential interference of neurotransmitter receptor function by elevated-CO ₂ , as previously observed in marine fishes. Altered behaviour of marine invertebrates at projected future CO ₂ levels could have potentially far-reaching implications for marine ecosystems.