A 15-year-old Schnauzer, showing right exophthalmos, was diagnosed as adenocarcinoma originated from the third eyelid gland. On computed tomography, a normal right third eyelid gland was not observed. Instead, a heterogeneous cystic mass invaded the retrobulbar space and displaced the eye globe dorsolaterally. In addition, lysis of the bony nasolacrimal duct was found, which was considered the tumor invasion. These findings indicated that third eyelid gland adenocarcinoma should be considered when a retrobulbar mass is found ventromedial to the globe without observation of the normal third eyelid and accompanies osteolysis of the bony nasolacrimal duct in dogs showing exophthalmos.