We report an incidental case of intravascular large B-cell lymphoma (IVLBCL) coexisting with an ovarian carcinoma in a 76-year-old woman. She visited our hospital with difficulty in defecation. Magnetic resonance imaging and computerized tomography scan revealed a solid and cystic mass probably arising from the left ovary. Gross examination of the tumor obtained by an exploratory surgery showed a solid area in a simple cyst. The ovarian tumor was diagnosed as a high-grade serous carcinoma (HGSC). Early in the post-operative course, this patient developed fever of unknown origin with central nervous system manifestations. Magnetic resonance imaging of the brain showed multiple space-occupying lesions. When we reviewed the histological sections, atypical lymphocytes were found in the lumina of small vessels of almost the entire ovary. These cells were positive for CD20 and CD79a by immunohistochemistry. A diagnosis of IVLBCL coexisting with HGSC was finally made. Although radiation therapy for brain lesions was performed and rituximab was administered, she died two months after the operation. To the best of our knowledge, this is the first case of IVLBCL incidentally identified in HGSC through microscopic examination. This case serves to create awareness of the rare event where IVLBCL may involve the ovary of patients who also have carcinoma in the organ.