Thalassemia is an inherited hemoglobinopathy characterized by anemia. In Japan, beta-thalassemia occurs in only 1 in 1000 individuals, and reports of thalassemia in patients with breast cancer are extremely rare. We report a case of triple-negative breast cancer in which thalassemia manifested as progressive anemia during neoadjuvant chemotherapy. A Filipino woman in her 40 s with a family history of breast cancer presented with a left breast mass. Physical examination revealed a 2-cm palpable mass in the outer lower quadrant of the left breast. Ultrasonography confirmed a 21-mm irregular hypoechoic mass in the corresponding area with immunohistochemistry indicating a triple-negative phenotype (ER-, PgR-, HER2-negative, Ki-67 index 60%). Pembrolizumab, paclitaxel, and carboplatin were administered every 3 weeks, during which the hemoglobin (Hb) level gradually decreased. Thalassemia was diagnosed based on low pretreatment mean corpuscular volume (67.6 fL), presence of target cells in peripheral blood, and elevated fetal hemoglobin (HbF) levels. Despite the anemia progression, the patient was able to complete the planned chemotherapy regimen with blood transfusion support. This included 4 cycles of pembrolizumab, doxorubicin, and cyclophosphamide. Subsequently, a partial mastectomy plus sentinel lymph node biopsy was performed, and the patient achieved a pathological complete response. This case demonstrates that neoadjuvant chemotherapy for breast cancer can be successfully completed with appropriate blood transfusion support in patients with thalassemia-induced anemia.