Abstract: Adverse childhood experiences (ACEs) may result in long-term mental health complications, including post-traumatic stress disorder (PTSD). ACEs are known to be more frequent among military personnel, despite their need to maintain their mental health to accomplish their missions. Self-compassion, or treating oneself with kindness and understanding, can mitigate the psychological effects of adversity but is also affected by adversity. This cross-sectional study aimed to identify the complex relationships between ACEs, self-compassion, and PTSD symptoms among 752 new recruits of the Japan Air Self-Defense Force, of whom 537 with ACEs completed the PTSD Checklist for DSM-5. Hierarchical multiple regression analysis was used to examine the independent effect of self-compassion, measured using the Self-Compassion Scale, on PTSD symptoms. Mediation effect analysis with self-compassion as a mediator was conducted on the relationship between ACEs and PTSD symptoms. We confirmed high levels of ACEs among our participants compared to a healthy population of a previous study, and approximately 6% presented PTSD symptoms above a threshold. Self-compassion was significantly negatively associated with PTSD symptoms (? = -.22, 95% confidence interval [CI], -.34 to -.11). Mediation effect analysis revealed that self-compassion partially mediated the relationship between ACEs and PTSD symptoms, explaining 6.9% of this effect, and ACEs were negatively associated with self-compassion (? = -.13, 95% CI, -.22 to -.04). These findings suggested that self-compassion is a protective factor against PTSD symptoms, whereas ACEs can decrease self-compassion. Further research should explore educational interventions to enhance self-compassion among individuals with ACEs to mitigate PTSD symptoms.