PURPOSE To describe a nasolacrimal duct (NLD) obstruction secondary to an ectopic tooth in a 5-year-old male Border collie. The dog was presented with a 1-month history of mucopurulent discharge from the left eye (OS) preceded by a lifelong history of epiphora OS. Treatment with neomycin/polymyxin B/dexamethasone ophthalmic solution had not improved the clinical signs, and the NLD was not patent when irrigated by the referring veterinarian. METHODS A complete ophthalmologic examination was performed followed by dacryocystorhinography and computed tomography (CT). RESULTS The ophthalmologic examination revealed marked mucopurulent discharge, mild conjunctivitis, slightly elevated STT measurements, and a negative Jones test OS. Both nasolacrimal puncta OS could be cannulated without resistance for approximately 1.5 cm. Upon irrigation, copious amounts of mucopurulent discharge were exited through the corresponding punctum, while no fluid could be detected at the nares. Dacryocystorhinography was performed. Radiographs revealed an ectopic left canine tooth within the left nasal cavity. A cystic dilation of the NLD was observed proximal to the ectopic tooth. Computed tomography was performed to determine the exact position of the tooth and possible involvement of adjacent structures; CT confirmed the previous imaging findings. Treatment with systemic antibiotics, NSAIDs, and ofloxacin ophthalmic solution led to resolution of the clinical signs within several days. Surgery was declined by the owner. CONCLUSION This is the first case report describing a blocked NLD due to an ectopic tooth in a dog. Ectopic teeth should be included as a differential diagnosis in cases of dacryocystitis and chronic epiphora in dogs.