Cushing syndrome is rare in infancy and usually due to an adrenocortical tumor (ACT). We report an infant with Cushing syndrome due to adrenocortical carcinoma. The patient presented at six months of age with a three-month history of growth failure, rapid weight gain, acne, and irritability. Physical examination showed obesity, hypertension, and Cushingoid features. Biochemical evaluation showed very high serum cortisol, mildly elevated testosterone, and suppressed ACTH. Abdominal MRI revealed a heterogeneous right adrenal mass extending into the inferior vena cava. Evaluation for metastases was negative. The tumor was removed surgically en bloc. Pathologic examination demonstrated low mitotic rate, but capsular and vascular invasion. She received no adjuvant therapy. Her linear growth has improved and Cushingoid features resolved. Hormonal markers and quarterly PET scans have been negative for recurrence 24 months postoperatively. In conclusion, adrenocortical neoplasms in children are rare, but should be considered in the differential diagnosis of Cushing syndrome.