Acceptance and Commitment Therapy (ACT) is a prominent form of psychological treatment which is used to treat an array of concerns, including emotional problems. The main focus of ACT is to increase psychological flexibility, which allows one to remain in the present moment and behave flexibly according to one’s values. A related topic is implicit theories of emotion, which refers to one’s belief that emotions can change (incremental theory) or are stable and not subject to change (entity theory). This study examined whether implicit theories of anxiety and emotion change the relationship between psychological flexibility and symptoms of depression and anxiety. Results indicated mediation, such that psychological flexibility worked through entity theories to predict greater symptomology. This suggests that targeting implicit theories may be important when treating depression and anxiety using ACT.