Since dissociative identity disorder (DID) has symptoms similar to schizophrenia, such as auditory hallucinations and delusional thoughts of being controlled, there are difficulties in its differential diagnosis. A 16-year-old adolescent male patient who was previously diagnosed with schizophrenia from a different hospital was admitted to our inpatient psychiatric unit for the evaluation of auditory hallucinations and suicide attempts. Through psychiatric evaluations, it was determined that the patient suffered from identity alternation, dissociation, and amnesia. As for the diagnostic evaluations, the following measures were implemented: a psychiatric interview regarding the diagnostic criteria, mental status examination, laboratory tests, brain imaging studies, electroencephalography, and full psychological test for adolescents, and the self-reported measure of the Adolescent Dissociative Experiences Scale. The patient was diagnosed with DID, and the following treatments were administered: pharmacotherapy, ego state therapy, psychoeducation regarding emotions, trauma-focused psychotherapy including stabilization, and family therapy. Following treatment, in the internal dimensions, the patient was able to recognize the nine alternate identities in charge of his emotions, which established a basis for the potential integration of identities. In the external dimensions, he showed improvements in the aspects of family conflicts and issue of school refusal. This is the first reported case of DID in an adolescent in Korea; it emphasizes the consideration of DID in the differential diagnosis of other mental illnesses such as schizophrenia, bipolar disorder, and posttraumatic stress disorder and expands the treatment opportunities for DID by sharing the procedures of ego state therapy.