Residual cysts are infrequent inflammatory cysts of the jaws of odontogenic origin and usually occur due to incomplete removal of radicular cysts. They are mostly asymptomatic and are detected incidentally on routine radiography. A 51-year-old male patient applied to our clinic with a painless swelling in the right maxillary region that started one year ago and grew over time. It was learned that the patient, who had no systemic disease in the anamnesis, had not undergone any surgical intervention or trauma other than a tooth extraction performed many years ago in the relevant region. The orthopantomography obtained showed a large volume unilocular radiolucent lesion with regular borders filling the right maxillary sinus and extending between the first premolar and second molar teeth. A straight incision was made over the crest of the patient under local anesthesia. An incisional biopsy was performed from the exposed cyst capsule for definitive histopathological diagnosis of the lesion and the cyst content was evacuated. Then, the decompression tube was sutured to the tissue and the cyst cavity was made open to the mouth. As a result of the 5-month follow-up, a significant decrease in cyst size was observed and enucleation was performed. It is treated surgically by enucleation, marsupialization, or decompression to reduce intraluminal pressure within the cyst. Enucleation aids in complete surgical excision of the cyst however, in some cases, the size of the cystic lesion, its location, proximity to adjacent anatomical structures, and the patient's age constitute a contraindication for enucleation. This study presented a case of a large-volume residual cyst of the maxillary sinus treated with decompression and secondary enucleation due to its rarity.