Organized hematoma (hematocele, blood boil) is defined as a hemorrhagic pseudotumor of unknown cause in the maxillary sinus. It is difficult to differentiate this lesion from a malignant tumor owing to similar features. We report the case of 65-year-old woman with an organized hematoma in the right maxillary sinus, which extended into the oral cavity. She visited our hospital to undergo a complete examination of swelling in the right upper gingiva. At the first visit, she presented with swelling of the right cheek and a feeling of mild nasal obstruction. Computed tomography and magnetic resonance imaging showed a tumor-like lesion filling the right maxillary sinus. The surrounding bone was displaced expansively and thin. A contrast study suggested that tumor had high blood flow. Histopathologically, a biopsy specimen demonstrated only necrotic tissue and scar tissue, with no tumor cells. On the basis of these findings, we diagnosed a benign tumor of the maxillary sinus and planned surgical enucleation of the tumor. Separation of the tumor from the surrounding tissue was uncomplicated, and there were no tumor cells in the excised specimen. Finally, we diagnosed an organized hematoma. The patient remains free of disease as of 4 years after surgery.