BACKGROUND: Cushing disease is a rare but serious endocrine disorder. It involves increased cortisol levels, which can damage the function of various systems throughout the body, including the central nervous system, affecting cognition, memory, and emotion. However, it is rare that Cushing disease presents as a mental disorder. This paper reports the complete diagnosis, treatment process, and follow-up of a patient with Cushing disease whose main symptom was bipolar disorder. CASE PRESENTATION: A 31-year-old woman was diagnosed with bipolar disorder 9 years ago and was admitted to hospital several times due to repeated episodes of depression and mania. In October 2017, after careful examination and consideration by doctors, the patient was diagnosed with Cushing syndrome and underwent transsphenoidal pituitary microadenoma resection. Glucocorticoid replacement therapy and antipsychotic therapy were administered postoperatively. After careful treatment and care by the medical staff, the patient's cortisol levels returned to normal, and her mental symptoms were significantly improved. CONCLUSION: This is a rare case of Cushing disease marked by psychiatric symptoms. Neuropsychiatric symptoms can precede the onset of Cushing disease, making diagnosis challenging.