Background: Different from rhinoliths, the paranasal gossypiboma is a foreign body, such as a surgical sponge, left in the nasal cavity. It is a rare, frequently misdiagnosed disease that has rarely been reported. We summarize its clinical characteristics, management, and possible risk factors. Methodology: We reviewed medical records of confirmed paranasal gossypibomas at a tertiary medical center between 2005 and 2015. Clinical symptoms, age, sex, anatomic sites, endoscopic photography, computed tomography, intraoperative findings, and past medical history were reviewed. Results: The study included 21 patients, each of whom had ultimately undergone two operations. Among them, 20 underwent endoscopic nasal surgery in primary hospitals, and 15 had been misdiagnosed during the second surgery. The average interval to discovery of a retained foreign body was 200 days. Predominant occurrence sites were the maxillary and ethmoid sinuses. Computed tomography showed paranasal gossypiboma as a heterogeneous cystic lesion with a thin "calcified shell. " Conclusions: A history of endoscopic nasal surgery, especially performed at a primary hospital, is a warning sign for clinicians. Computed tomography can add to the warning by showing a heterogeneous cystic lesion with a thin "calcified shell. " Clinicians should be aware of these characteristics to avoid misdiagnosing paranasal gossypiboma.