OBJECTIVE: To investigate the relationship between preinjury or early personal resources and long-term psychosocial outcomes following moderate-to-severe traumatic brain injury (TBI) and examine evidence for the stability of personal resources over time. METHODS: The review protocol was registered with the International Register of Systematic Reviews (PROSPERO, Registration No. CRD4202341056). A search of PsycINFO, Cumulative Index to Allied Health Literature (CINAHL), MEDLINE, Scopus, and Web of Science was conducted from inception to February 23, 2023, for longitudinal studies involving adults with moderate-to-severe TBI that examined: (1) the relationship between preinjury or early personal resources (measured ≤6 months postinjury) and later psychosocial outcomes or (2) stability of personal resources over time with a minimum reassessment interval of 3 months. Two reviewers independently assessed eligibility and rated methodological quality of studies using a checklist informed by Strengthening the Reporting of Observational Studies in Epidemiology. RESULTS: A narrative synthesis was conducted on 14 eligible articles summarizing 12 studies (N = 826). Nine studies examined the impact of preinjury or early personal resources on long-term psychosocial outcomes, most typically at 12 months postdischarge. Out of 9 studies 7 indicated that self-reported preinjury or early personal resources, including productive coping, higher self-esteem and resilience, and lower neuroticism, were associated with better psychosocial outcomes. Evidence from 7 studies examining the stability of personal resources over time was generally mixed, with personality changes (eg, neuroticism, conscientiousness, and extraversion) more evident from informant ratings than self-ratings. CONCLUSION: Preinjury or early personal resources may influence later psychosocial outcomes after TBI. Further research is needed to investigate the stability of personal resources and factors mediating or moderating change across the adjustment trajectory.