--- - |2+ Background There has been a strong association between posttraumatic stress disorder (PTSD) and suicidal ideation, which are both major mental health concerns in firefighters. Alcohol use disorders (AUDs) and insomnia are linked with both PTSD and suicidal ideation, but no studies have examined whether the relationship between PTSD and suicidal ideation can be explained by AUDs and insomnia. The purpose of this study was to investigate the mediating role of AUDs and insomnia in the relationship between PTSD symptoms and suicidal ideation. Methods A total of 7190 Korean firefighters filled out self‐reported questionnaires to assess the number of exposure to incident stressors and the severity of PTSD symptoms, suicidal ideation, AUDs, and insomnia. Hierarchical multivariable linear regression analyses were performed to identify the relationship of AUDs and insomnia with suicidal ideation. Path analyses were applied to investigate the mediation effects of AUDs and insomnia on the relationship between PTSD symptoms and suicidal ideation. Results AUDs and insomnia showed significant associations with suicidal ideation, even after adjusting for demographic factors, number of traumatic events, and PTSD symptoms. The relationship between PTSD symptoms and suicidal ideation was partially mediated by AUDs and insomnia. AUDs also had both direct and indirect effects on suicidal ideation, with the indirect effect mediated by insomnia. Conclusions We presented a model in which AUDs and insomnia mediate the relationship between PTSD symptoms and suicidal ideation in firefighters. Efforts to treat AUDs and alleviate insomnia could be beneficial in minimizing suicidal ideation in firefighters. - Depression and Anxiety, EarlyView.