Breast tumors in adolescents are very rare and mostly benign. Fibroadenomas are the most frequent, but within the extensive differential diagnosis, the phyllodes tumor must be mentioned, which accounts for about 1% of breast tumors and the diagnosis of which is very rare in patients younger than 20 years. There are no specific symptoms or radiological images to distinguish phyllodes tumor from fibroadenoma; therefore, histological examination is mandatory for diagnosis. Histology also allows the classification of phyllodes tumor into benign, borderline, or malignant types for appropriate surgical treatment: freemargin excision in benign tumors and mastectomy in the other two types. Fortunately, the majority of these tumors are benign, and treatment maximizes breast conservation with free infiltration margins surgery, given that this fact is the most important factor to prevent local recurrence. In this article, we describe a rare case of borderline cystosarcoma phyllodes in a 12-year-old girl.