A referred patient presented with a lesion of endodontic origin located at the apex of tooth #27. The tooth had been endodontically treated and re-treated. A periapical radiograph revealed a close relationship between the lesion and the maxillary sinus. A cone-beam computed tomography scan confirmed that the lesion had invaded the sinus cavity. The treatment plan consisted of periapical surgery using an endoscope as a magnification device. Due to a sinus membrane perforation, a new sinus membrane repair technique was performed. Twelve months after surgery, a cone-beam computed tomography scan revealed successful healing of the lesion. The continuous preservation of the sinus physiology was also observed. The use of an endoscope as a magnification device and a tailored technique for sinus membrane management allowed us to achieve a successful treatment outcome in the case of an endodontic lesion invading the maxillary sinus.