An 8-year-old, female chipmunk (Tamias sibiricus), which had been kept as a pet, was presented with a chief complaint of the gradually enlarging mammary mass and dysuria. The mammary mass was surgically excised and was histopathologically diagnosed as mammary adenocarcinoma. Despite a general improvement in its condition and reduced dysuria after surgery, the chipmunk died on postoperative day 188. Pathological examination revealed that the mammary tumor had metastasized to both the lungs and the pelvic cavity. The metastatic focus in the pelvic cavity involved the left ureter, with ipsilateral hydronephrosis. Immunohistochemically, the tumor cells were stained positive for cytokeratin (CK) AE1/AE3 and partially positive for CK7, but negative for CK20. This is the first report of a mammary tumor in chipmunks.