The purpose of this study was to develop a sport-specific self-report measure of metacognitive beliefs: The Metacognitive Beliefs about Performances Questionnaire (MBPQ). Firstly, principal components analysis and reliability tests showed that the MBPQ retained a reliable five-factor solution: positive beliefs about competitive worry; positive beliefs about rumination; positive beliefs about arousal; negative beliefs about competitive thinking; and beliefs about need to control cognition. Secondly, confirmatory factor analysis, reliability and validity tests confirmed internal validity in a triathlete field sample. Thirdly, concurrent validity was also evident through analyses of bivariate correlations with existing generalised metacognitive measures (the Metacognitions Questionnaire 30; the Adapted Positive Beliefs about Rumination Scale), and through associations with state anxiety variables (cognitive anxiety, somatic anxiety, thought disruption, and self-confidence). Overall, the results have conveyed the MBPQ to be a sound measure of metacognitive beliefs in competitive athletes. The implications and interactions of these subscales were discussed accordingly.