BACKGROUND: Primary mediastinal seminomas most commonly occur in young men, and they are localised in the anterior mediastinum. CASE PRESENTATION: The presented study is a case report of a 34-year-old man suffering from a mediastinal tumour in size of 19 cm, with pleural and pericardial effusion. The patient complains of cough, difficulty breathing, weight loss, and pronounced tiredness. CT of lungs and biopsy of the mediastinal change was performed. The histopathological analysis was in favour of a primary mediastinal seminoma. The patient initially had pronouncedly increased levels of LDH and β-hCG tumour marker. Pericardiocentesis was realised due to threatening tamponade of the heart, followed by 4 cycles of chemotherapy by BEP protocol. Following 2 cycles of chemotherapy, normalisation of LDH and β-HCG levels and significantly improved the clinical condition in the patient was found. Upon completion of 4 chemotherapy cycles by BEP protocol, the patient performed an FDG-PET scan with partial response and reduced dimension of a primary tumour in the mediastinum. Radiotherapy of residual tumour mass up to a total dose of 40Gy in 20 fractions was realised. Control FDG-PET scan had a finding of complete response to a tumour and absence of FDG uptake. The last follow-up examination was performed in October 2018, and the patient was disease-free for 54 months. CONCLUSION: Multimodality treatment approach of chemotherapy followed by radiation consolidation ensured long-term survival in primary advanced mediastinal seminoma.