Bilateral macronodular adrenocortical disease as a part of McCune Albright Syndrome (MAS) is the most common cause of endogenous Cushing's syndrome (CS) in infancy. Adrenocortical tumors causing CS in infancy are extremely rare. We report the case of a girl with CS who presented at age 4 months with obesity and growth retardation. Her 8 am paired cortisol and adrenocorticotropic hormone levels were 49.3 µg/dL and <1 pg/mL, respectively with non-suppressed serum cortisol (41 µg/dL) on high-dose dexamethasone suppression test. Abdominal computed tomography scan demonstrated a 5.3x4.8x3.7 cm homogenous left adrenal mass with distinct borders. Laparotomy following pre-operative stabilization with ketoconazole 200 mg/day, revealed a 7.5x5x4 cm lobulated left adrenal mass with intact capsule and weighing 115 grams. Histopathology showed small round adrenal tumor cells with increased nucleo-cytoplasmic ratio and prominent nucleoli. The cells were separated by fibrous septae without any evidence of vascular or capsular invasion- findings consistent with adrenal adenoma. On the 8th post-operative day, after withholding hydrocortisone supplementation, the 8 am cortisol level was <1 µg/dL, suggestive of biochemical remission of CS. The patient improved clinically with a 7.5 kg weight loss over the next 3.5 months. This is perhaps the youngest ever reported infant with CS due to adrenal adenoma. Lack of clinical and biochemical evidence of hyperandrogenism as well as the benign histology in spite of the large tumor size (>7 cm diameter; 115 g) are some of the unique features of our patient.