What is a Rheumatologist? A rheumatologist is a specialist who is expert in the diagnosis and treatment of arthritis and other diseases of the joints, muscles and bones. They have three years of training in either internal medicine or pediatrics and an additional two years in specialized rheumatology training. What do rheumatologists treat? Rheumatologists treat arthritis, autoimmune diseases, musculoskeletal pain disorders and osteoporosis.