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. [ABSTRACT FROM AUTHOR]