Mediastinal Seminoma is one of the malignant germ cell tumors of the mediastinum. It is a rare tumor that represents only 1%-4% of mediastinal tumors. We report the case of a patient treated at the Oncology Center of Oujda for a pure primitive seminoma of the mediastinum, with a review of the literature to assess the epidemiological, clinical, therapeutic, and prognostic characteristics of this pathology. This is a 25-year-old man whose aunt is being treated for breast cancer and who complained of persistent dry cough with asthenia without any other associated signs. The initial imaging revealed a voluminous locally advanced tumor process in the anterior and middle mediastinum. Tumor markers Alpha-fetoprotein (AFP) and beta-subunit of chorionic gonadotrophic hormone (BHCG) were normal, as well as testicular ultrasound. The CT- guided transthoracic biopsy with anatomopathological study and immunohistochemistry was in favor of a pure seminoma. The treatment consisted of primary chemotherapy with BEP protocol. The response was favorable with a significant reduction in tumor size estimated at 90%. The residual tumor was inoperable due to its intimate contact with the vascular structures and was treated with external radiotherapy at a total dose of 36 Gy. The evolution at 20 months after treatment was in favor of a good clinical and radiological evolution.