Sinus augmentation is a routine procedure performed in patients presenting with severe atrophy of the posterior maxillary alveolar ridge who seek dental implant surgery. Although this strategy is successful in large part, complications such as maxillary sinusitis, antral bleeding, resorption of graft material, infection, failure of implant installation, and oroantral fistula formation are documented on occasion. However, reports of postoperative maxillary cyst (POMC) arising after sinus augmentation are rare. Described herein is a rare and large case of POMC formation after sinus augmentation. A brief review of the pertinent literature is included.