Neoplasms of the conjunctiva and third eyelid gland are rare ocular lesions in dogs. Adenocarcinoma, melanoma, squamous cell carcinoma, papilloma, mast cell tumor, lymphoma, hemangioma, myoepithelioma, extramedullary plasmacytoma, malignant peripheral nerve sheath tumors and basal cell carcinomas are tumors reported in the third eyelid and are generally invasive. Mast cell tumors have a high prevalence in cats, but have a very low incidence in dogs. These tumors, which are specific to the skin, are rarely seen in primary extra cutaneous tissues and they cause variable size swelling due to mast cell degranulation and histamine release in the affected tissues. The case was a 6-year-old male, Kangal dog who was brought to Istanbul University, Cerrahpasa Veterinary Faculty Clinic. The dog was brought to our clinic with complaints of redness, pain and ocular discharge in the right eye for a long time. Ophtalmologic examination revealed follicular conjunctivitis and a large, firm swelling with protrusion of the third eyelid. Incision was made 2-3 mm from the bulbar conjunctival surface of the third eyelid parallel to the edge of the eyelid and the mass was removed with tear gland by total excisional biopsy. Incisional conjunctival mucosa was closed with 5/0 polyglactin 910 suture material with simple continuous suture. The mass was soft and fleshy, 1X 1,5X 0,5 cm in diameter with no significant capsule formation and histopathological examination revealed mast cell tumor of third eyelid gland. Topical and systemic medical treatment was applied for two weeks following the operation. It was learned that the clinical complaints of the patient disappeared completely after the treatment. No recurrence occurred during the 6-month post-operative period. In this case report, it is aimed to share the clinical and histopathological examination findings and treatment results of mast cell tumors of the third eyelid gland which is rare in dogs.