Abstract: We report an extremely rare case of malignant mesothelioma in the tunica vaginalis testis. A 67-year-old man was referred to our hospital complaining of painless swelling in the right scrotum. Ultrasonography and computed tomography demonstrated a mass of approximately 6 cm in the right scrotum. He underwent tumor resection under a diagnosis of right intrascrotal tumor. The histopathological diagnosis was malignant mesothelioma, and he died 26 months later from multiple metastases. Asbestos may be a significant contributor to malignant mesothelioma in Japan. There have been 46 cases reported in urology departments in this country of the related symptoms, but only two of these have been clearly attributable to asbestos, which may be due to the investigations being insufficient to obtain a definitive diagnosis. In addition, this symptom is often diagnosed preoperatively as hydrocele testis; it is therefore important to carry out ultrasonography when a hydrocele testis has been diagnosed.