Adenoid cystic carcinoma (ACC) of the breast with high-grade transformation (dedifferentiation) is an extremely rare entity in the literature. Due to its potential to cause significant diagnostic challenges, recognition of this variant is clinically important. We report the case of a 58-year-old woman presenting with a 7  6 cm retroareolar breast mass. Core biopsy revealed features consistent with classic ACC; however, intraoperative frozen section of a sentinel lymph node demonstrated poorly differentiated carcinoma, showing no resemblance to the initial biopsy. This rare case is presented to contribute to the literature and raise awareness of this unusual phenomenon in breast tumors.