A bitch was presented for investigation of the mass in left 5th mammary gland. The partial mastectomy was performed and submitted for the histopathological diagnosis. The mammary mass was firm and white colored. The cut surface was separated with several lobules and developed vessels. The central area of the mass formed the cavity filled with inflammatory exudates. The dominant component of the tumor was the bundles of spindle-shaped cells. Some tumor cells possessed atypical nuclei and were arranged in a solid nest. Cysts were microscopically composed of hemorrhage, necrosis, and exudates, partially surrounded by tumor cells and granulation tissues. Histopathologically, the mammary mass revealed spindle cell carcinoma. The bitch made a complete recovery following the mastectomy. This case was a rare mammary spindle cell carcinoma in a dog.