Tibor Jack Greenwalt (January 23, 1914 – July 17, 2005) was an American hematologist who specialized in transfusion medicine. Greenwalt earned his medical degree from New York University Medical School in 1937 and completed a hematology fellowship under William Dameshek. After serving as a US Army physician in World War I, he settled in Milwaukee, Wisconsin, where he became a professor of medicine at the Marquette University School of Medicine. He was a founding member of the American Association of Blood Banks and served as president of the International Society of Blood Transfusion from 1966 to 1972.