Borderline personality disorder and externalizing disorders are associated with suicide-related behaviors. The present study examined whether symptoms of borderline personality disorder mediate the relationship between externalizing disorders and suicide-related behaviors. Diagnostic interviews were administered to 344 participants (n = 233 women). Results indicated that symptoms of antisocial personality disorder, alcohol use disorders, and drug use disorders each were significantly associated with suicide threats and self-injurious behavior in women and symptoms of antisocial personality disorder were associated with suicide attempts in women. With the exception of the association between symptoms of alcohol dependence and self-injurious behaviors, borderline personality disorder symptoms mediated or partially mediated all associations between externalizing disorders and suicide-related behaviors in women. These results highlight the importance of assessment and treatment of borderline personality disorder symptoms in individuals with externalizing disorders, particularly in the presence of suicide-related behaviors.