Theory and research point to the role of attachment difficulties in borderline personality disorder (BPD). Attachment insecurity is believed to lead to chronic problems in social relationships, attributable, in part, to impairments in social cognition, which comprise maladaptive mental representations of self, others, and self in relation to others. However, few studies have attempted to identify social–cognitive mechanisms that link attachment insecurity to BPD and to assess whether such mechanisms are specific to the disorder. For the present study, empirically derived indices of mentalization, self–other boundaries, and identity diffusion were tested as mediators between attachment style and personality disorder symptoms. In a cross-sectional structural equation model, mentalization and self–other boundaries mediated the relationship between attachment anxiety and BPD. Mentalization partially mediated the relationship between attachment anxiety and antisocial personality disorder (PD) symptoms, and self–other boundaries mediated the relationship between attachment anxiety and avoidant PD symptoms. The findings support theories that insecure attachment is associated with difficulties in social cognition and that a distinctive pattern of impairment characterizes BPD.