Testicular tumours have many different manifestations, including hydrocele formation. Herein, we present an extremely rare case of testicular mesothelioma presenting with left hydrocele, but without risk factors. Left radical inguinal orchidectomy was performed, and pathological examination revealed a malignant mesothelioma of the tunica vaginalis of the testis. No infiltration of the spermatic cord was evident, and upon advanced radiological evaluation, no sign of metastasis was detected. Follow-up was still ongoing in our urology outpatient clinic at the time of this report. Although hydrocele is a simple and common condition that is easy to diagnose, a detailed investigation should be performed. Thus, when encountering a patient with hydrocele, the clinician should evaluate the possibility of the presence of an underlying testicular/paratesticular tumour, including a rare one such as mesothelioma of the tunica vaginalis.