Extranodal natural killer/T-cell lymphoma comprises less than 1% of all non-Hodgkin lymphomas. It is rare in Western countries but is common in East Asia and Central and South America. The pathological features are angiocentricity /angioinvasion and significant tissue necrosis. A 72-year-old woman was diagnosed with primary breast extranodal natural killer/T-cell lymphoma. The patient presented with a painless right breast tumor and had uneven internal echo and strip blood flow signal on breast ultrasonography. After right breast tumor resection, the pathological diagnosis was extranodal natural killer/T-cell lymphoma. Despite receiving CHOP chemotherapy, the patient died of lymphoma and multiple organ dysfunction syndrome 27 months after diagnosis. Extranodal natural killer/T-cell lymphoma with breast tissue as the primary site is very rare. The disease is prone to misdiagnosis and missed diagnosis, and diagnosis by ultrasound is difficult, so pathological examination after biopsy is particularly important.