The present study investigates indirect effects of communication about suicide in interpersonal relationships and patient-provider relationships on suicidal intention via attitude, descriptive norm, injunctive norm, and self-efficacy in committing suicide among Korean adults, using longitudinal survey data. Moderation effects of perceived social support are also tested. As a result of path analysis (N = 984), communication about suicide with family, friends, and coworkers is significantly related to pro-suicide descriptive norm and pro-suicide injunctive norm. Positive attitude toward suicide, pro-suicide injunctive norm, and high self-efficacy in committing suicide are significantly related to suicidal intention. Interpersonal communication is positively related to pro-suicide injunctive norm, which in turn links to suicidal intention. Communication about suicide with healthcare professionals, however, did not show significant direct and indirect effects. Moderation effects of perceived social support are detected between interpersonal communication and pro-suicide attitude and pro-suicide injunctive norm, as well as between patient-provider communication and pro-suicide attitude.