Leiomyosarcoma is malignant mesenchymal tumor of smooth muscle and commonly encountered in the gastrointestinal tract of dogs. However, primary canine leiomyosarcoma in oral cavity is rare due to lack of smooth muscle in the oral tissue. A 13-year-old, neutered male Poodle presented a hard and immobile mass on the left maxilla. Imprinting cytology from the mass as well as fine-needle aspirated cytology from the left scapular lymph node revealed predominant spindle cells met malignancy criteria of the tumors, including coarse chromatin, high N/C ratio, nuclear molding, macro/multi-nucleoli with cigar-shaped nucleus. Radiography of the skull showed lysis of the nasopalatine bone, and mineral radiopacity in the mass. Computed tomography showed soft tissue attenuating mass from the left incisor teeth to the left retrobulbar space with loss of nasopalatine bone and medial wall of orbit. The histopathological examination showed irregularly arranged malignant spindle-shaped cells with oval or elongated nuclei. The nucleolus is distinct and moderate cellular polymorphism is observed. Mitotic figures are occasionally observed. The tumor cells are positive to vimentin, desmin, α-smooth muscle actin when immunohistochemistry was performed, and in Masson’s trichrome stain, tumor cells are stained as red. Overall, histopathologic exam and immunohistochemistry confirmed canine oral leiomyosarcoma. Because of the poor prognosis, the owner did not consent further treatment.