Background: Mucinous tumors are typically benign lesions, although they can be aggressive and exhibit stromal invasion. Moreover, studies have reported on extremely large mucinous ovarian malignancies that have infiltrated the abdomen and pelvis. Case Report: The authors describe the history of a 52-year-old woman who presented with a 40-cm mucinous ovarian neoplasm that weighed nearly 22 kg. Initially, confirming the diagnosis was complicated and thus, immunohistochemistry (e.g., pancytokeratin, p53, PAX 8, and CK7) staining was indicated. Conclusion: Mucinous ovarian tumors are often associated with a favorable prognosis, but when they are extremely large, disease management can be relatively precarious because of the potential for cardiovascular and intraoperative complications. Since there is some difficulty in assessing large mucinous tumors, immunohistochemistry staining and judicious clinical judgment are essential.