Mucinous cystadenomas account for approximately 10-20% of all epithelial ovarian tumours. About 75-80% of these tumours are benign [1]. They are usually seen between the third and fifth decades and 5-10% of them are bilateral. These benign tumours are rare entity in adolescents. The majority of ovarian masses in adolescent patients are non-epithelial in origin, with a predominance of germ cell tumours, while epithelial neoplasms make up a small proportion of ovarian masses in this age group [2,3]. Here we report a giant ovarian mucinous cystadenoma in an adolescent, the seventh largest mucinous cystadenomas reported in the literature. A 13 year old, premenarchal, previously healthy girl presented to the emergency department of our hospital with complaints of vomiting, constipation and abdominal pain for past one day. The patient also complained of slowly increasing abdominal distension and mild abdominal discomfort since past four months. Patient was taken up for laparotomy. A cystic solid pelvic mass was present originating from right ovary. Unilateral salpingoophorectomy with tumor removal was performed. The tumor measured 25x20x15 cm and weighed 3.5 kilograms. Specimen was sent for histopathological examination and diagnosis of mucinous cystadenoma was made.