Abstract A castrated, mixed‐breed, 10‐year‐old male dog developed a skin fistula from a ruptured periorbital swelling on the right eye; the patient was successfully treated with dacryoendoscope‐guided cannulation using a nasolacrimal tube. On presentation, computed tomography with concurrent dacryocystography (CT‐DCG) revealed obstruction of the right nasolacrimal duct and abnormal flow of contrast agents towards the root of tooth 109 at the right maxillary bone; however, the origin of the fistula remained unclear on CT‐DCG. Dacryoendoscopy was used to confirm the physical association between the fistula and the nasolacrimal duct; the fistula end was found to be near the right upper gum mucosa around tooth 107. CT‐DCG and dacryoendoscopy revealed that the periorbital swelling occurred owing to an infection or inflammation of the accessory nasolacrimal duct. The fistula end was connected to the oral cavity through a nasolacrimal tube installed in the oral mucosa. The tube was removed a week later, and the clinical symptoms completely resolved.