Acinic cell carcinoma (ACC) of the breast is a rare subtype of breast cancer, and its clinicopathological and molecular characteristics remain poorly understood. We report a case of breast ACC detected through pathological examination of a risk-reducing mastectomy (RRM) specimen in a carrier of a BRCA1 pathogenic variant. The patient was a woman in her 30s who underwent neoadjuvant chemotherapy for right-sided breast cancer, followed by right total mastectomy and contralateral RRM. Histopathological evaluation of the RRM specimen revealed ACC of the breast. No adjuvant therapy was administered, and the patient has remained disease-free for two years after surgery. Reports of breast ACC associated with BRCA1 pathogenic variants are limited. This case highlights the extreme rarity of breast acinic cell carcinoma occurring in BRCA1 pathogenic variant carriers and emphasizes the need for further case accumulation to clarify its clinicopathological features and potential genetic background.