Self-discrepancy occurs when a person feels the failure to fulfill one’s hopes or responsibilities. Although self-discrepancy has been widely examined to elucidate patients’ chronic pain adjustment, the underlying mechanism is unclear. The present study proposes that the effect of self-discrepancy on pain outcomes is accounted for by psychological inflexibility, which involves the psychological processes that guide behaviors in the pursuit of goals and values. One-hundred patients with chronic pain were recruited from a public hospital. They were invited to participate in a semi-structured interview regarding their self-discrepancy and complete self-reported questionnaires regarding their psychological inflexibility and pain outcomes. The results confirmed that psychological inflexibility partly accounts for the variance observed between self-discrepancy and pain outcomes. The current study provides additional insight into the mechanism underpinning the impact of self-discrepancy on patients’ pain adjustment and offers clinical implications regarding the use of acceptance commitment therapy for chronic pain management.