Intravascular lymphomatosis (IVL) is a rare type of B-cell non-Hodgkin lymphoma (BCNHL), which can mimic many other diseases. Fever, neurological symptoms, and skin findings are the most frequent clinical findings. Intravascular lymphomatosis may be associated with genetic factors and infection with human immunodeficiency virus (HIV), human herpesvirus 8 (HHV-8), human T-lymphotropic virus 1 (HTLV1), Epstein-Barr virus (EBV), and hepatitis B virus (HBV). A 50-year-old man was hospitalized with recalcitrant hyponatremia of unknown cause. He had also telangiectatic, indurated, slightly erythematous plaques on his trunk for the last 10 days. His past medical history was unremarkable, although he was a carrier of hepatitis B. Multiple skin biopsies were performed and were considered to be diagnostic of IVL. The hyponatremia was unresponsive to water restriction and hypertonic solution support but it resolved with B cell directed chemotherapy. A final diagnosis was made as syndrome of inappropriate antidiuretic hormone (SIADH) in the setting of IVL. This case had a relatively early diagnosis with just 10-days of skin lesions. Intravascular lymphomatosis is a very rare disease and is usually difficult to diagnose. An even more uncommon presentation is IVL complicated by a paraneoplastic syndrome. There are prior reported cases of SIADH in the setting of IVL. However, this case underscores the importance of evaluating patients with SIADH for potential IVL.