Granulocytic sarcoma (GS) is a rare extramedullary tumor composed of immature myeloid cells. It is usually associated with leukemia or other myeloproliferative disorders. It occurs very rarely without overt hematologic diseases. A 19-year-old man presented with left knee mass. Biopsy with pathological analysis showed lymphoma aspect. Immunostains yielded the diagnosis of GS with myeloperoxidase and CD43 positivity. There was no systemic manifestation of leukemia, and bone marrow biopsiy was negative for neoplastic infiltration. Chemotherapy by CHOP was efficient, and the patient remaind alive and healthy 40 months after the end of treatment. The case is discussed in the framework of the existing literature about the diagnosis, treatment, and prognosis of this very rare condition.