A primary mucosa-associated lymphoid tissue lymphoma (MALT lymphoma) of the breast is extremely rare. We report a case of a MALT lymphoma of the breast that presented as a palpable left breast mass in a 37-year-old woman. A physical examination revealed a large firm, and fixed mass in the left inner breast. Mammograms showed a large, isodense mass in the lower inner quadrant of the left breast and an enlarged lymph node in the axilla. A sonogram demonstrated a 5 cm sized, oval, circumscribed, and heterogeneously hypoechoic mass with posterior acoustic enhancement. A surgical biopsy was performed, and the pathology revealed a MALT lymphoma.