45,X/46,XY mosaicism is a sex development disorder with an estimated incidence of less than 1 in 15,000 live births. Various studies have shown there is an increased risk of germ cell tumours forming in Mosaic Turner syndrome. This includes gonadoblastoma, a clinically benign mixed germ-stromal cell tumour. However, this can later develop into one or several malignant germ cell neoplasms, for which early prophylactic gonadectomy is often recommended in patients with 45,X/46,XY mosaicism. The study presents the case of an 11-year-old patient diagnosed with a Mosaic Turner syndrome karyotype, who underwent prophylactic bilateral gonadectomy.