OBJECTIVE: Our study examines the association between suicidal ideation and and self-reported symptoms of posttraumatic stress disorder (PTSD), major depressive disorder (MDD), generalized anxiety disorder (GAD), and alcohol use disorder (AUD) in a sample of treatment-seeking Canadian combat and peacekeeping veterans; and identifies potential predictors of suicidal ideation. METHODS: Actively serving Canadian Forces and Royal Canadian Mounted Police members and veterans seeking treatment at the Parkwood Hospital Operational Stress Injury Clinic (n = 250) completed measures including the Primary Care Evaluation of Mental Disorders Patient Health Questionnaire, the Alcohol Use Disorder Identification Test, and the PTSD Checklist-Military Version (PCL-M) between January 2002 and December 2010. Regression analyses were used to determine the respective impact of PTSD, and self-reported symptoms of MDD, GAD, AUD, and anxiety on suicidal ideation. RESULTS: Most people met PCL-M screening criteria for PTSD (73.6%, n = 184), while 70.8% (n = 177) screened positively for a probable major depressive episode. PTSD symptom was significantly associated with suicidal ideation (β = 0.412, P < 0.001). After controlling for self-reported depressive symptom severity, AUD severity, and generalized anxiety, PTSD severity was no longer significantly associated with suicidal ideation (β = 0.043, P = 0.58). CONCLUSIONS: Although PTSD alone is associated with suicidal ideation, after controlling for common comorbid psychiatric illnesses, self-reported depressive symptom severity emerged as the most significant predictor of suicidal ideation. These findings support the importance of screening for comorbidities, particularly an MDD, as potentially modifiable conditions that are strongly related to suicidal ideation in military personnel's endorsing criteria for PTSD.