Here we present a transgender male adolescent with an androgen receptor-positive serous borderline ovarian tumour in the setting of testosterone treatment for medical gender transition. To our knowledge, this is the second report of borderline tumour in a transgender individual and the first in an adolescent, an age group in which borderline tumours are extremely rare. We discuss the specific considerations of treating ovarian tumours in the transgender male population, the incompletely understood role of androgens in the genesis of ovarian epithelial neoplasia, and an emphasis on assessing cancer risk in transgender patients based on patient anatomy.