Primary meningeal neoplasias are rare, and their diffuse growth within the subarachnoid space may simulate inflammatory processes or metastatic neoplastic dissemination. We report here the case of a boy with an 18-day history of fever and progressive neurological deterioration. While in the hospital, the patient showed neurological deterioration and did not respond to antituberculosis treatment. His cerebrospinal fluid (CSF) cytology disclosed an elevated white blood cell count accompanied by a mildly elevated protein level and a slightly decreased glucose level. The child died, and pathology revealed that his meningeal process was a sarcoma. The immunophenotype of the neoplastic cells showed expression of a sarcomeric actin marker, characterizing rhabdomyoblastic differentiation of the cells that occupied the subarachnoid space and invaded superficially the encephalon. Rhabdomyoblastic differentiation in leptomeningeal diffuse primary sarcoma (LDPS) is an aspect rarely observed in this malignant meningeal neoplasia, with few reported cases. The present case is the first reported in the Portuguese literature and the fifth reported in the English literature.