BACKGROUND Mediastinal masses can originate from anatomical structures normally located in the mediastinum, or from structures that travel through the mediastinum during embryogenesis. Initial presenting symptoms usually vary from shortness of breath, cough, chest pain, and superior vena cava syndrome to nonspecific constitutional symptoms (eg, fever, weight loss, fatigue). However, the initial presentation of a mediastinal mass with acute pericarditis has not been reported in the literature as far as we know. CASE REPORT A 20-year-old man presented to the Cardiology Clinic with chest pain and new pericardial effusion on echocardiography, both fulfilling the diagnostic criteria of acute pericarditis. The patient also had venous engorgement on the neck, and a chest X-ray followed by computed tomography imaging showed a large mediastinal mass. The serum tumor marker a-fetoprotein (AFP) was markedly elevated. The biopsy and immunohistochemistry revealed a high-grade malignant neoplasm - yolk sac tumor, which is a type of non-seminomatous germ cell tumor. The acute pericarditis resolved after administration of NSAID and colchicine. The patient was then started on chemotherapy. CONCLUSIONS The discussed case shows the rare presentation of an anterior mediastinal mass with acute pericarditis. This emphasizes the importance of a thorough review of systems and critical analysis of every sign and symptom at the time of initial presentation, which helps the physician to obtain appropriate imaging studies early in the course, leading to an early diagnosis and treatment of the disease, such as in this case of an extremely rare germ cell tumor.