Objective: To outline overlap and boundaries between ICD-11 definitions of complex post-traumatic stress disorder (C-PTSD) and personality disorder (PD) and propose guiding principles that may assist practitioners in assigning one or both of the two diagnoses. Conclusions: The ICD-11 definitions for C-PTSD and PD are substantially comparable in terms of self- and interpersonal problems, and childhood trauma may be at the root of both disorders. The ICD-11 formally recognizes this overlap and allows the assignment of both diagnoses at the same time. The C-PTSD diagnosis essentially differs from a PD diagnosis by requiring a history of trauma and PTSD symptoms. Moreover, C-PTSD typically involves stable and persistent patterns of negative self-perception while emphasizing avoidant interpersonal patterns. In comparison, the PD diagnosis may differ from C-PTSD by allowing an unstable or internally contradictory sense of self, which may involve both overly negative and overly positive self-views. When the diagnostic requirements for both C-PTSD and PD are met, only the C-PTSD diagnosis should be assigned, unless the PD diagnosis may contribute with clinically useful information that is not sufficiently covered by the C-PTSD diagnosis. The outlined similarities and boundaries must be further corroborated by future empirical studies.