The behavioural effects of elevated carbon dioxide (CO₂) levels projected to occur in the oceans by the end of the century in fish has been extensively studied, but whether the behaviours of other highly active marine organisms, such as cephalopods, are similarly affected is unknown. In this study, we tested the behavioural effects of projected future CO₂ levels (626 and 956 µatm) on the two-toned pygmy squid, Idiosepius pygmaeus. Exposure to elevated CO₂ increased the number of individuals that were active as opposed to at rest by 19-25% and increased movement (number of line-crosses) in those that were active by nearly 3 times compared to control squid kept at present-day CO₂ levels. The vigilance and defensive behaviours of the squid were also altered by elevated CO₂ with over 80% of individuals reacting with jet escape responses over defensive arm postures in response to a visual startle stimulus, compared with 50% choosing the jet escape responses at control CO₂. In addition, more escape responses were chosen over threat behaviours in regard to the body pattern displays at elevated CO₂ and individuals were more than twice as likely to discharge ink as an escape strategy at the highest CO₂ treatment, compared with controls. The increased activity seen here could lead to adverse effects on energy budgets as well as increasing visibility to predators. A tendency to respond to a startle stimulus with escape behaviours could increase survival, but may also be energetically costly and could potentially lead to more chase responses by predators compared with individuals that use defensive postures. These results demonstrate for the first time that projected future ocean acidification levels affect the behaviours of cephalopod species.