Rationale: Ectopic tooth is tooth erupting out of normal anatomical position. Ectopic tooth can occur in different positions, such as maxillary sinus and nasal cavity. In this article, we present a rare case of an ectopic tooth with a dentigerous cyst in the maxillary sinus compressing the nasolacrimal canal. Patient concerns: An 8-year-old girl presented with a 2-month history of spontaneous lacrimation in her right eye. When she wept, more tear shed from her right eye than that from the left one. Computed tomographic (CT) imaging showed a huge low-density image containing a tooth in the maxillary sinus in her right maxilla; the right nasolacrimal canal vanished due to the compression of the ectopic tooth. Diagnoses: Ectopic tooth with dentigerous cyst of right maxilla, and obstruction of nasolacrimal duct Interventions: The patient underwent nasal endoscopic maxillary sinus cystectomy. Outcomes: The patient recovered well after cystectomy and has been symptom-free. Lessons: The unique finding is that this is the first report about ectopic tooth compressing the nasolacrimal canal and inducing spontaneous lacrimation. Treatment: aspect: surgery under endoscope is a minimally invasive approach to ectopic tooth.