An elderly man presented to the emergency department with shortness of breath, peripheral edema, and significant weight loss. Blood tests revealed anemia and elevated inflammatory markers, and chest imaging showed a massive left pleural effusion. During hospitalization, he developed subacute cardiac tamponade, and pericardiocentesis was performed. Further imaging revealed a primary malignant cardiac tumor with extensive infiltration of the cardiac tissue, and biopsy was deemed impossible due to the tumor’s location. The most likely diagnosis was angiosarcoma. The cardiac surgery team evaluated the case and considered it inoperable due to the tumor’s extensive infiltration. The patient is currently under the regular care of a palliative care team. This case underscores the difficulties of diagnosing primary cardiac tumors, particularly in elderly patients with comorbidities. Despite advances in imaging and surgical techniques, the prognosis for malignant cardiac tumors remains poor.