Described herein is a case of primary breast lymphoma (PBL) that underwent spontaneous regression (SR). A 71-year-old woman visited hospital because of the rapid growth of a tumor in her left breast. On imaging, including magnetic resonance imaging (MRI), a discrete solid nodule was detected, which suggested malignant tumor. Histology of the following core needle biopsy (CNB) specimen indicated diffuse large B-cell lymphoma (DLBCL). The patient had no past history of lymphoma and there was no evidence of systemic lymph nodes enlargement. After CNB, however, the patient noticed that her breast nodule gradually decreased in size without any specific treatment. Subsequent MRI showed an ill-defined nodular area suggesting a regressing tumor. Excisional biopsy indicated fibrotic mammary tissue devoid of large neoplastic lymphoid cells. The patient has remained well without evidence of recurrent lymphoma more than 18 months after her original diagnosis. This case is considered to be unique in a PBL showing SR, probably induced by an intervention of CNB and histologically confirmed on sequential examinations in addition to illustrative before-and-after imaging. To the best of the authors' knowledge no other PBL of DLBCL has been reported as undergoing a complete SR in the English-language literature.