Complex dissociative disorders (CDD) include dissociative identity disorder (DID) and the most common other specified dissociative disorder (OSDD, type 1). One of the strongest predictors of CDD is antecedent trauma, particularly early childhood trauma. Currently, consensus-based treatment guidelines for CDD are lacking. Schema Therapy (ST) is a psychotherapeutic approach that has been recently proposed as a treatment for CDD given its emphasis on the consequences of early childhood neglect and abuse, and the explanation within the therapeutic model of the patient’s experience of drastic shifts between personality states. The present case study examined the process of individual ST, in a three phases-based approach, with a 38-year-old male Arabic-speaking refugee with OSDD, type 1 (i.e., chronic dissociative disorder with mixed symptoms), posttraumatic stress disorder (PTSD) and borderline personality disorder (BPD). The setting included two therapists (co-therapy) and an interpreter/cultural mediator. We assessed the patient’s change by using self-report assessments of dissociative and PTSD symptoms, cognitive schemas over 2-year and 4-months treatment periods and a 6-months follow-up. We also assessed the therapeutic alliance. Posttreatment and follow-up reliable change analyses showed significant improvements in dissociative and PTSD symptoms as well as in some cognitive schemas. The therapeutic alliance between the patient and the therapists grew in a progressive and balanced way. This case study shows that ST within a phase-oriented approach may be an effective treatment for CDD. Our study also suggests cross-cultural validity of ST and, particularly, of modes construct. More research based on large samples is needed to confirm these assumptions.