Although changes to self-identity or self-discrepancy are common after severe traumatic brain injury (TBI), the mechanisms underlying these changes are poorly understood. This study aimed to examine the influence of personality characteristics and cognitive appraisals on self-discrepancy. Participants were 51 adults (74% male) with severe TBI (M age = 36.22 years; SD = 12.65) who were on average 34 months (SD = 40.29) post-injury. They completed self-report measures of personality style (optimism and defensiveness), cognitive appraisals (threat appraisals, rumination and reflection and perceived coping resources), and self-discrepancy (Head Injury Semantic Differential Scale – III). Correlation analyses identified that higher levels of optimism, defensiveness and perceived coping resources were significantly associated with more positive self-discrepancy (r = .29−.47, p < .05), whereas higher threat appraisals and rumination were significantly related to more negative self-discrepancy (r = −.50−.57, p < .001). After controlling for personality characteristics, cognitive appraisals significantly accounted for self-discrepancy (R2 change = .15). Moreover, rumination significantly mediated the relationship between optimism and self-discrepancy. In summary, cognitive appraisals were found to be related to self-discrepancy, independent of personality characteristics. Rumination in particular may be an important target of psychological intervention for individuals experiencing negative self-discrepancy.