An 8-year-old, neutered male Yorkshire Terrier dog presented with left ventromedial canthus swelling over a one-month period, refractory to pharmacological therapy. There was no history of trauma. On ultrasonography, the lesion had a cystic character filled with anechoic fluid and hyperechoic sedimenting debris. The computed tomography (CT) and CT-dacryocystography showed a cystic lesion protruding from the lacrimal sac fossa and occupying a defect in the orbital plate and an ethmoidal ectoturbinate surrounded by a bony structure with an intact nasolacrimal system. The dog underwent the surgical resection of the cyst and its fluid content was aspirated. Ethmoid mucocele was diagnosed based on the CT, cytologic examination, bacterial culture and histopathologic findings. This case describes the imaging characteristics of an ethmoid mucocele and highlights the importance of CT and CT-dacryocystography in dogs with ventromedial canthus swelling that had poor response to medical treatment.