Abstract: Combat-exposed veterans and military service members are at high risk of experiencing mental health problems. Although moral injury is recognized as a unique contributor to mental health challenges in this population, the relationship between changes in world belief, PTSD, and substance use remains unexplored. This cross-sectional quantitative study investigated the relationship among moral injury symptoms, changes in belief in a just world (BJW) and belief in an unjust world (BUW), and substance use in a sample of 122 combat-exposed U.S. military veterans and service members. Hierarchical regression analyses revealed that changes in BUW following potentially morally injurious events (PMIEs) significantly predicted greater moral injury symptom severity, above and beyond PTSD, age, and gender (ΔR² = .105, p = .003). Additionally, increased BUW predicted PTSD, while changes in BJW did not. Mediation analyses using the PROCESS macro indicated that neither changes in BJW nor BUW significantly mediated the relationship between moral injury and substance use, despite a significant association between moral injury symptoms and BUW. These findings are supported by existing literature, suggesting that exposure to PMIEs challenges core beliefs about justice and morality, contributing to moral and psychological distress. These findings could better prepare mental health practitioners to provide existing interventions or create new, targeted interventions to improve mental health outcomes for veterans and military personnel.