Turner syndrome is a chromosomal disorder that occurs in an estimated 1 in 2500 female live births. It is estimated that 6%-12% of all Turner syndrome patients will be a mosaic with Y-chromosomal elements putting them at risk for gonadoblastoma and subsequent dysgerminoma. While 30%-50% of this population demonstrate gonadoblastoma, we only found 23 reported cases of dysgerminoma in the literature, and no reported cases of seminoma. We present the first case of seminoma in a phenotypic Turner 15-year-old female after prophylactic gonadectomy.