A rapidly growing mass was observed in the left third mammary gland of a 5-and-a-half-year-old intact female northern tree shrew (Tupaia belangeri). Histopathologically, the mass was composed of numerous irregularly dilated mammary ducts with a pericanalicular or intracanalicular pattern. These mammary ducts showed a leaf-like structure due to the proliferations of surrounding spindle-shaped neoplastic cells with moderate to severe nuclear atypia. We diagnosed this case as a phyllodes tumor in a mammary gland of a northern tree shrew (Tupaia belangeri), which closely resembled those occurring in the human breast histopathologically. To the best of our knowledge, this is the first report of a phyllodes tumor in the mammary gland of a northern tree shrew (Tupaia belangeri).