Primary thymic carcinoma is a rare and heterogeneous group of tumours of the anterior mediastinum that includes different histological types. Frequently, it first comes to clinical attention through paraneoplastic syndromes such as dermatomyositis. We report the case of a 54-year-old woman with several episodes of fever and chest pain in the previous 5 months. The patient was admitted to the Rheumatology Department with a peri-ungual erythema and papular lesions on both sides of each hand and alteration at biochemistry tests. A left deltoid muscle biopsy specimen showed a histological pattern compatible with the diagnosis of dermatomyositis. A computed tomography of the chest, abdomen and pelvis, showed a solid mass in the upper anterior mediastinal area and a mediastinoscopy with mass-biopsy was performed. Only the immunohistochemical staining technique allowed a definitive histological diagnosis. We report the diagnostic challenge and the therapeutic approach of thymic neoplasia.