The most common cause of ectopic Cushing's syndrome is small cell lung cancer; less common causes include pancreatic and thymic neuroendocrine tumors. A 35-year male was investigated after detecting low potassium in the tests performed for weakness. The patient was admitted for exclusion of Cushing's syndrome because of high cortisol (108 µg/dl) and ACTH (827ng/L) levels. There was no suppression in the high-dose dexamethasone test, and the patient was thought to have ectopic Cushing's syndrome. A mass in the thymus was detected in thorax tomography. Postoperative ACTH and cortisol levels decreased rapidly. Postoperatively, ACTH did not drop to normal, suggesting the possibility of residual tumor. Radiotherapy was given to the patient because the surgical margin was positive in the pathology report. No functional focus was detected in Ga 68 DOTATATE PET CT after radiotherapy. This case is presented because of the rare association of a thymic neuroendocrine tumor with ectopic Cushing's syndrome, which was revealed during the investigation of the etiology of hypokalemia. Key Words: Hypokalemia, Cushing syndrome, Thymic neuroendocrine tumor.