Cognitive processes, in particular, erroneous beliefs surrounding gambling outcomes and abilities, are thought to play an important role in the development and maintenance of disordered gambling behaviours. The current study investigated the impact of erroneous gambling-related cognitions on gambling severity and intentions to gamble, trialing a brief, online cognitive intervention designed to reduce gambling-related cognitions and intentions to gamble. One hundred and twenty one participants were randomly allocated to either an educational intervention condition or an alternative intervention (control condition). Participants in the educational intervention condition showed larger reductions in gambling-related cognitions than did participants in the control condition. The educational intervention significantly reduced erroneous cognitions from pre to post treatment, with effectiveness of the intervention increasing in those exhibiting more severe gambling behaviours.