Robert Hugh Ferrell (May 8, 1921 – August 8, 2018) was an American historian and a prolific author or editor of more than 60 books on a wide range of topics, including the U.S. presidency, World War I, and U.S. foreign policy and diplomacy. One of the country's leading historians, Ferrell was widely considered the preeminent authority on the administration of Harry S. Truman, and also wrote books about half a dozen other 20th-century presidents. He was thought by many in the field to be the "dean of American diplomatic historians," a title he disavowed.