Objective: The process model of emotion regulation posits beliefs about emotions inform regulation processes, affecting mental-health outcomes. Beliefs that emotions can be controlled (i.e., control beliefs, underpinning traditional Cognitive Behaviour Therapy) and accepted (i.e., acceptance beliefs, underpinning Acceptance and Commitment Therapy), though seemingly opposing, are both associated with lower psychological distress. This study tested the hypothesis that emotion regulation flexibility (i.e., using and applying a range of regulation strategies at the right times) may mediate the relationship between these beliefs and distress. Method: 177 participants (87.6% female, Mage = 42.7) completed the 21-item Depression, Anxiety and Stress Scale, the Emotion Belief Questionnaire-General Controllability Composite, and two measures developed for this study: the Emotion Belief Questionnaire-Acceptance Variant and Emotion Regulation Flexibility Questionnaire. Results: Emotion regulation flexibility mediated the relationship between control and acceptance beliefs and distress, such that stronger control and acceptance beliefs were associated with lower distress via higher emotion regulation flexibility. Conclusions: Our results are consistent with theorising that both control and acceptance beliefs are useful for mental health; these beliefs may inform usage of control or acceptance-based emotion regulation strategies, which may reduce distress levels if applied flexibly. These findings are discussed regarding corresponding therapeutic interventions. Key Points What is already known about this topic: (1) The process model of emotion regulation posits beliefs about emotions inform emotion regulation processes and mental health outcomes. (2) Control and Acceptance beliefs while seemingly opposing are both associated with lower psychological distress. (3) Emotion regulation flexibility is associated with lower psychological distress. What this topic adds: (1) Having both high control and acceptance beliefs is associated with lowered psychological distress via higher emotional regulation flexibility. (2) Flexibly shifting beliefs is associated with lowered psychological distress. (3) Inflexible controls beliefs is not associated with psychological distress, while Inflexible acceptance beliefs is associated with psychological distress.