Military psychiatric inpatients with and without a lifetime history of non-suicidal self-injury (NSSI), combined with a history of at least one suicide attempt, were compared on suicide ideation severity, number of suicide attempts, and Interpersonal-Psychological Theory of Suicide variables. Data were derived from baseline assessments performed in a psychotherapy randomized controlled trial. Lifetime history of NSSI and lifetime number of suicide attempts were assessed using the Columbia Suicide Severity Rating Scale (C-SSRS; Posner et al., 2011Posner, K., Brown, G. K., Stanley, B., Brent, D. A., Yershova, K. V., Oquendo, M. A., … Mann, J. J. (2011). The Columbia–suicide severity rating scale: Initial validity and internal consistency findings from three multisite studies with adolescents and adults. American Journal of Psychiatry, 168(12), 1266–1277. doi:10.1176/appi.ajp.2011.10111704). Individuals with versus without a combined lifetime history of attempted suicide and NSSI showed significant elevations on thwarted belongingness and acquired capability for suicide. No significant between-group differences were found on perceived burdensomeness, frequency, duration, and controllability of suicide ideation, or number of lifetime suicide attempts. A history of NSSI, above and beyond attempted suicide, appears to increase service members’ social alienation and acquired capability for suicide.