Torsion of the hernia sac is a rare disease that presents as acute scrotum in children. Including the present case, only six cases have been reported in the English literature. We report a 10-year-old boy who presented with pain and swelling of his right scrotum. Ultrasonography revealed a hypoechoic region adjacent to the normal right testis. The inflammatory changes of the right scrotum deteriorated. The patient underwent surgery and a necrotic cyst was recognized within a hydrocele of the scrotum. The cyst was not connected with the testis or epididymis and was twisted at an angle of 270 degrees. The cause of the necrotic cyst observed was anatomical and pathological torsion of the hernia sac.