A patient with multiple endocrine neoplasia type 1 (MEN1) developed a mediastinal seminoma. The patient was a 46-year-old man who presented with respiratory symptoms. A diagnosis of mediastinal seminoma was pathologically confirmed and a complete remission was achieved by chemotherapy. During his hospital stay, hyperparathyroidism and multiple pancreatic tumors associated with hypergastrinemia were found. A diagnosis of MEN1 was made genetically. Although patients with MEN1 manifest a variety of neoplastic disorders, no cases of concurrent seminoma and MEN1 have previously been reported. In addition, no etiological relationship between seminoma and MEN1 has yet been reported.