Breast carcinoma is a very rare disease in children. We present a rare case of breast cancer in an 11-year-old prepubertal girl. Clinically, it was a case of locally advanced breast cancer (T4bN1M0). The core biopsy report showed adenocarcinoma of the not otherwise specified (NOS) variety (oestrogen receptor-negative, progesterone receptor-negative and human epidermal growth factor receptor 2-negative). Diagnosis was late in this case due to lack of suspicion. A modified radical mastectomy was considered to be adequate treatment. Histologically, it was adenocarcinoma NOS, which is rare in young girls (the secretory type being more common). Incidence, differential diagnoses, investigation and management of breast carcinoma in young girls are discussed. The purpose of reporting this case is to highlight that prevention and early detection of breast carcinoma in children is very important.