BackgroundPrimary mediastinal seminoma is rare, especially with complications of superior vena cava syndrome (VCSS) and large thrombus.Case presentationA 23-years-old Indonesian male complained of dyspnea, phlegm cough, chest pain, and body weight loss. The patient experienced swelling in the face and neck 1 week ago, accompanied by increased jugular venous pressure. Radiological results showed a firm mass in the right area of the mediastinum. Tumor marker and IHC results showed mediastinal seminoma. Electrocardiography showed sinus tachycardia, right axis deviation, and V1-V5 slow R wave progression. Echocardiography showed an intracardiac mass (RA protrusive RV, size 7.2 × 3.8 cm) with an intracardiac thrombus and RV failure. The patient was positioned in a semi-Fowler's position and given furosemide 3 × 20 mg, dexamethasone 3 × 5 mg, and warfarin 1 × 4 mg. Meanwhile, the results of the biopsy revealed a malignant germ cell tumor. When the patient was going to have bronchoscopy and radiotherapy planned, the patient died.DiscussionReporting primary mediastinal seminoma cases with complications of VCSS and large thrombus has a high risk of mortality, so this report can be used as a review to improve management in future.ConclusionMediastinal seminoma with complications has a high mortality.