Fibroadenoma is the most common benign mass of breast in females 20-50 years of age. Juvenile fibroadenoma accounts for only 4% of total fibroadenomas. Moreover, the incidence of giant juvenile fibroadenoma is found to be only 0.5% of all fibroadenomas. A 15-year-old woman at the 12th week of gestation was referred to the Shahid Sadoughi Hospital, Yazd, Iran because of a mass in the right breast from one year ago. On Physical examination and imaging, a large mass was found in her right breast. Fine needle aspiration biopsy reported fibroadenoma. Mass enucleation and breast reconstruction were performed. Pathologic examination confirmed a giant fibroadenoma. No serious complications were reported during one-month follow up. The most highlighted point of this presentation was giant size of juvenile fibroadenoma in a pregnant woman. This case report shows that large breast tumors in the adolescent age group can be benign and breast-conserving surgery should be considered in such cases.