Extrinsic compression of the pulmonary arteries by mediastinal masses, while rare, is the most common cause of non-congenital peripheral pulmonary artery stenosis. Granulocytic myeloid sarcomas are an extramedullary manifestation of acute myeloid leukemia, with a wide variability in their presentation. They can present either de novo as soft tissue masses or in conjunction with acute myeloid leukemia. Myeloid sarcomas are uncommon and their prognosis is generally considered poor when present in patients with acute myeloid leukemia (AML). We present a case of a granulocytic myeloid sarcoma causing pulmonary artery stenosis and right ventricular failure. This case represents a unique cause of pulmonary arterial stenosis.