Loneliness impacts both physical and psychological health and is associated with increases of all-cause mortality and suicidal behavior. Because loneliness may result from a variety of developmental, interpersonal, and intrapersonal factors, distinguishing its components, origins, and sustaining factors as it manifests in various psychopathological states are important steps in formulating interventions to alleviate these conditions. To date, loneliness has not been widely studied in relation to complex posttraumatic stress disorder (PTSD), which is newly delineated in the International Classification of Diseases, characterized by PTSD symptoms in the context of significant early trauma, as well as "disturbances in self-organization" marked by affective dysregulation, negative self-concept, and disturbances in relationships. In this article, illustrating with case material, we suggest that loneliness plays a major role in the development of complex PTSD and in the preservation of its symptoms. Consequently, therapies for complex PTSD should include interventions that address loneliness.