Oscar Davis Ratnoff (August 23, 1916 – May 20, 2008) was an American physician who conducted research on the process of coagulation and blood-related disorders. Ratnoff discovered the substance later known as factor XII and was one of the primary contributors to the delineation of the exact sequence that makes up the clotting cascade. He also made notable research contributions to the understanding of the complement system and to the detection and treatment of hemophilia.