Purpose and Objective: Psychological distress is known to contribute to recovery following mild traumatic brain injury (mTBI) and there is a need to understand the mechanisms that contribute to this relationship. The present study examined psychological flexibility, as a hypothesized psychological mechanism, in 169 treatment-seeking adults with mTBI. Research Method/Design: Participants completed self-report measures of postconcussion symptoms, psychological distress (anxiety, stress, and depression) and functional status within four weeks of entry to an mTBI outpatient clinic. A general measure (Acceptance and Action Questionnaire), as well as a context-specific (Acceptance and Action Questionnaire-Acquired Brain Injury) measure of psychological flexibility were administered. Results: Simple linear regression analysis showed that psychological flexibility made a significant contribution to the prediction of postconcussion symptoms and functional status. A series of multiple mediation analyses also found that psychological flexibility had a significant indirect effect on the relationships between psychological distress and postconcussion symptoms, and functional status. The context-specific, rather than the general measure of psychological flexibility, was consistently shown to contribute to these findings. Conclusions/Implications: These results suggest that psychological flexibility is a psychological mechanism that contributes to recovery outcomes in individuals with mTBI and could therefore be an important treatment target in mTBI interventions. (PsycInfo Database Record (c) 2021 APA, all rights reserved).