Ectopic development of teeth in nondental areas is uncommon, especially in the maxillary sinus. A panoramic radiograph is the routine diagnostic radiographic examination performed for this type of eruption, although cone beam computed tomography (CBCT) is highly recommended for further localization of the ectopic tooth and assessment of the characteristics of any associated lesion before a surgical procedure. We report a case of a 13-year-old female student who presented with purulent discharge posterior to the upper right second molar with a bad taste and foul odour. Radiographic examination revealed a maxillary third molar tooth located at the posterosuperior aspect of the right maxillary sinus with a hyperdense lesion surrounding the crown, obliterating the sinus cavity. Both the tooth and dentigerous cyst were surgically removed under general anaesthesia through Caldwell-Luc antrostomy. After a three-month follow-up, the patient was symptom free and had an uneventful recovery. The rare and critical location of the reported third molar along with the infected dentigerous cyst indicates its complete enucleation to avoid complications as recurrence or malignant transformation.