Objective: This study investigated the associations between triathletes’ sports-specific metacognitive beliefs (i.e., the Metacognitive Beliefs about Performances Questionnaire), metacognitive processes (i.e., the Metacognitive Processes during Performances Questionnaire) measured prior to a triathlon (n = 193), and in-event flow (i.e., the Short Flow State Scale) measured post event (n = 76). Method: The Metacognitive questionnaires were administered to participating triathletes one day prior to the event, and the flow scale was administered just following the event. Bivariate correlations were used to test relationships with individual flow dimensions, while stepwise regressions were used to determine the strongest metacognitive predictors of meta processes and flow. Results: Correlations indicated that metacognitive beliefs were negatively associated with various specific dimensions of flow (Cohen’s f 2 = .28), while metacognitive processes positively associated with flow dimensions (Cohen’s f 2 = .49). Stepwise regressions revealed that specific metacognitive beliefs were negatively associated with metacognitive processes during competition (Cohen’s f 2 = .08 to .49), including the coordination, evaluation and control of cognition. Further stepwise regressions demonstrated that negative beliefs about competitive thinking, thought control, and cognitive coordination predicted experience of flow during competition. Conclusions: Overall, this study demonstrated that sports specific metacognitive beliefs and processes may influence the regulation of flow during a competition, however, further research using longitudinal and qualitative methodologies is required to understand the relationships on a deeper level.