Insecure attachment style and borderline personality disorder (BPD) symptoms are related to the severity of alcohol use disorder (AUD) and can also complicate its course and treatment. We propose a theoretical model of relationships between adult attachment style and BPD and AUD symptom severity in which the sense of identity is a mediator. The aim of our study was to test this model in a sample of male inpatients with AUD. Male inpatients with AUD (N = 114) aged 22–72 years (M = 43.05; SD = 9.84) completed the Experiences in Close Relationships-Revised, the Multidimensional Identity Questionnaire, the Alcohol Use Disorders Identification Test and the Borderline Symptom List 23. Results showed that the sense of identity fully mediated the positive relationship between attachment anxiety and BPD symptom severity. There was also a positive indirect effect of attachment avoidance on BPD symptoms via weaker sense of identity. Contrary to hypothesis, AUD symptom severity was not related to attachment, identity or BPD symptoms in our sample. Sense of identity could be a possible target for interventions for insecurely attached men with AUD to reduce BPD symptoms.