Malignant lymphomas of the breast are rare and primary breast lymphoma comprises <0.5 % of breast malignancies, within which T-cell lymphomas are an even rarer subset. We report a case of primary breast peripheral T-cell lymphoma not otherwise specified (PTCL-NOS). Histology of the biopsied specimen revealed CD2(+), CD3(+), CD4(+), CD5(-), CD7(+), CD8(-), CD20(-), CD25(-), CD30(+), CD56(-), bcl-2(-), EBV-ISH(-), TIA-I(-), and ATLA negative. The patient was treated with six cycles of the CHOP regimen and died 17 months after the diagnosis was made, despite complete remission after conventional chemotherapy. To our knowledge, only 18 cases of primary peripheral T-cell lymphoma of the breast and just one previous case of primary PTCL-NOS of the breast have been reported in Japan.