We report a rare case of a mediastinal malignant granular cell tumor in a 41-year-old man presenting with dyspnea, retrosternal chest pain, and recurrent pericardial effusion under treatment with corticosteroids. Because of recurrent episodes, further investigations with chest MRI and F-FDG PET/CT revealed a large infiltrating and strongly hypermetabolic mass in the mediastinum with pericarditis. Histopathology and immunohistochemical analysis showed large cells with greater pleomorphism and eosinophilic granular cytoplasm with positive staining for S100 protein and CD68 as well as high Ki67 proliferative index (15%) confirming the diagnosis of a malignant granular cell tumor.