Rituximab, sold under the brand name Rituxan among others, is a monoclonal antibody medication used to treat certain autoimmune diseases and types of cancer. It is used for non-Hodgkin lymphoma, chronic lymphocytic leukemia (in non-geriatric patients), rheumatoid arthritis, granulomatosis with polyangiitis, idiopathic thrombocytopenic purpura, pemphigus vulgaris, myasthenia gravis and Epstein–Barr virus-positive mucocutaneous ulcers. It is given by slow injection into a vein. Biosimilars of Rituxan include Blitzima, Riabni, Ritemvia, Rituenza (F.K.A. Tuxella), Rixathon, Ruxience, and Truxima.

Common side effects which often occur within two hours of the medication being given include rash, itchiness, low blood pressure, and shortness of breath. Infections are also common.

Severe side effects include reactivation of hepatitis B in those previously infected, progressive multifocal leukoencephalopathy, toxic epidermal necrolysis, and death. It is unclear if use during pregnancy is safe for the developing fetus or newborn baby, but it is not proven harmful.

Rituximab is a chimeric monoclonal antibody against the protein CD20, which is primarily found on the surface of immune system B cells. When it binds to this protein it triggers cell death.

Rituximab was approved for medical use in 1997. It is on the World Health Organization's List of Essential Medicines. Rituximab is co-marketed by Biogen and Genentech in the U.S., by Hoffmann-La Roche in Canada and the European Union, Chugai Pharmaceuticals, Zenyaku Kogyo in Japan and AryoGen in Iran.