Rheumatoid factor (RF) is an autoantibody, most commonly IgM class, that binds to the Fc region of IgG immunoglobulins. It is produced by plasma cells in the synovium (joint lining) and lymph nodes in people with autoimmune conditions, particularly rheumatoid arthritis (RA). RF is one of the serological markers used in the diagnosis of RA alongside anti-CCP antibodies.

RF is not specific to rheumatoid arthritis and can be positive in other conditions including Sjogren's syndrome, other autoimmune diseases, chronic infections, viral hepatitis, and in a small percentage of healthy older adults. Anti-CCP is more specific for RA than RF alone. RF positivity combined with anti-CCP positivity strongly supports RA diagnosis and predicts more erosive disease.