Background: Ewing sarcoma (ES) with ectopic adrenocorticotropic hormone (ACTH) syndrome (ectopic ACTH) is extremely unusual. This report details the first case in English literature of pediatric ES involving the proximal aspect of the humerus with florid ectopic ACTH. Case Report: A 9-year-old girl presented with mooning of the face and abdominal distension for the past 5 months with an unremarkable history. Her serum cortisol level measured at 8 AM was 42 µg/dL (reference range, 4.3-22.4 µg/dL). Serum ACTH level of 225 pg/mL (reference range, 10-46 pg/mL) suggested ACTH-dependent Cushing syndrome. Her serum cortisol level after the overnight dexamethasone suppression test was 60 µg/dL (reference value, <1.8 µg/dL), suggesting nonsuppressibility. The high-dose dexamethasone suppression test was nonsuppressible, suggesting ectopic ACTH secretion. Findings of magnetic resonance imaging of the brain were normal. Chest x-ray demonstrated a lytic lesion in the left humerus. Magnetic resonance imaging and 3-dimensional computed tomography scans of the left shoulder showed an expansile lesion in the proximal aspect of the humerus. A tru-cut bone biopsy with histopathology and immunostaining revealed clusters of small round cells with a mitotic index of 6/10 hpf to 8/10 hpf. CD99 staining confirmed ES. Ketoconazole was initiated. She received 1 cycle of chemotherapy with cyclophosphamide, vincristine, and doxorubicin and succumbed to the illness 1 week after chemotherapy. Discussion: Ectopic ACTH in the pediatric age group is rare and, coupled with the underlying etiology as ES, makes this case unique. Only 4 cases of ectopic ACTH with ES have been previously reported in the tibia, retroperitoneum, ischiopubic rami, and ribs. This is the first case of ES to have its origin in the humerus with ectopic ACTH. Conclusion: This case highlights an atypical presentation of ectopic ACTH caused by ES arising from the humerus. The etiology of ectopic ACTH as ES was confirmed by chance radiographic evidence of a lytic humerus lesion rather than symptoms.