Psychological research has shown that intentions to dope are one of the strongest predictors of doping behaviour, and attitudes are a strong predictor of intentions. One of the most relevant pending questions is when attitudes better predict intentions. The present study analysed the moderating role of meta-cognitive certainty on the relationship between attitudes and behavioural intentions related to doping. One hundred and sixty-six athletes (87.9% males, and 12.1% females) from different sports participated in a cross-sectional online study. They reported their attitudes and behavioural intentions regarding doping itself, as well as attitudes and behavioural intentions regarding a legalisation proposal. As expected, attitudes predicted intentions in both cases. Most importantly, we hypothesised and found an interaction between attitudes and attitude certainty on intentions regarding the legalisation proposal. That is, attitudes towards the legalisation predicted intentions to support that proposal and to engage in the banned behaviours if legalised to a greater extent for participants with higher (vs. lower) certainty in those attitudes. However, results regarding attitudes towards doping and intentions to dope were not conclusive and further research is necessary. These findings provide a new contribution to the study of meta-cognition in psychological research on doping, but future studies should examine and replicate our results in other samples (e.g., doping users). In conclusion, we suggest that future research and interventions related to doping could benefit from the use of attitude certainty measures as a moderator of the relationship between attitudes and behavioural intentions related to doping.