Background: Childhood trauma is negatively associated with depression severity in bipolar disorder; however, the underlying mechanisms remain unclear. We investigated whether personality traits (neuroticism, extraversion, openness, agreeableness, conscientiousness) mediate the relationship between childhood trauma and the severity of bipolar depression. Methods: Data from 209 individuals with bipolar disorder recruited for the Prechter Longitudinal Study of Bipolar Disorder were analysed. Using structural equation modelling, we examined the direct and indirect associations between childhood trauma (Childhood Trauma Questionnaire) and depression severity (Hamilton Depression Rating Scale) – with the personality traits (NEO Personality Inventory–Revised) as mediators. Results: The direct effect of childhood trauma on depression severity (standardised β = 0.32, 95% bootstrap confidence interval [CI] = 0.20–0.45, p < 0.001) and the indirect effect via neuroticism (standardised β = 0.03, 95% bootstrap CI [0.002, 0.07], p = 0.039) were significant; supporting a partial mediation model. The indirect effect accounted for 9% of the total effect of childhood trauma on depression severity (standardised β = 0.09, 95% bootstrap CI [0.002, 0.19], p = 0.046). The final model had a good fit with the data (comparative fit index = 0.96; root mean square error of approximation = 0.05, 90% CI = [0.02, 0.07]). Conclusion: Personality traits may be relevant psychological mediators that link childhood trauma to a more severe clinical presentation of bipolar depression. Consequently, a person’s personality structure may be a crucial operative factor to incorporate in therapeutic plans when treating individuals with bipolar disorder who report a history of childhood trauma.