A residual cyst is a specific type of radicular cyst that arises due to incomplete enucleation of granulation tissue, the cyst capsule, or the persistence of an undiagnosed cyst following the extraction of the causative tooth. Due to its asymptomatic nature, it may remain undetected for a long time. It is usually diagnosed incidentally through radiological examinations. A characteristic feature of a residual cyst is its tendency to grow despite the removal of the causative tooth. Similar to a radicular cyst, the origin of a residual cyst is the remnants of Malassez's epithelial rests located in the periodontal ligament affected by an inflammatory process. Residual cysts can reach significant sizes and may expand into adjacent anatomical spaces, such as the maxillary sinuses, nasal cavity, or mandibular canal. Within the maxillary sinuses, other cystic changes may also occur, such as mucoceles, pseudocysts, postoperative maxillary cysts, and retention cysts. Among the aforementioned pathological changes within the maxillary sinuses, the most frequently detected is the retention cyst. This study discusses the diagnosis and surgical treatment of an extensive residual maxillary cyst and a retention cyst of the maxillary sinus located on the medial wall of the right maxillary sinus in a 61-year-old patient.