A previously healthy 20-year-old woman presented with subacute onset of dyspnea and chest pain. Chest computed tomography scan revealed a contrast-enhanced anterior mediastinal mass (A, arrow) and circumferential pericardial (A, arrowheads) and pleural (A, asterisk) effusions. Pericardial and pleural fluid analysis ruled out autoimmune or infectious processes. Cardiac magnetic resonance imaging demonstrated a large cardiac mass with heterogeneous first-pass contrast-enhanced perfusion with gadolinium (B, arrow) and delayed-enhancement (C), right atrial invasion (D, arrow,Online Video 1), and large circumferential pericardial (E) effusion. Findings were suggestive of a highly vascular malignant tumor. Cardiac catheterization showed no right coronary artery involvement but an extensive network of feeding vessels to the mass (F,Online Video 2). Tumor debulking was attempted but unsuccessful due to the spread throughout the pericardium, right atrium, and atrioventricular groove (G). Cardiac pathological examination revealed an aggressive high-grade angiosarcoma with sheet-like growth pattern (H); CD31 showed membranous staining in the tumor cells (I). Chemotherapy with paclitaxel was started.