Intravascular large B-cell lymphoma (IVLBCL) is a subtype of diffuse large B-cell lymphoma, where the neoplastic lymphoid proliferation resides predominantly within the lumens of blood vessels but with no or few circulating neoplastic cells in the peripheral circulation. Focal or subtle involvement in some cases can cause the diagnosis to be misinterpreted or even overlooked, delaying the initiation of appropriate treatment. Our report focuses on a 78-year-old woman with a progressively enlarging thyroid mass, verified by ultrasound. She underwent a hemithyroidectomy, and microscopic evaluation demonstrated nodular thyroid parenchyma with atypical large cells in an intravascular distribution pattern identified on high magnification. Thorough evaluation showed that the large intravascular cells were positive CD20, PAX-5, and Ki-67 by immunoperoxidase staining, which lead to the diagnosis of IVLBCL. This case emphasizes the subtle appearance of IVLBCL, which may be missed on low-power light microscopy, and the need for careful evaluation of thyroid resection specimens.