Jacob Rader Marcus (1896–1995) was a scholar of Jewish history and a Reform rabbi.
Born in Connellsville, Pennsylvania, United States into a traditional Jewish family and raised in Homestead, Pennsylvania, Marcus became interested in Reform Judaism at the age of 15. At that time, he travelled to Hebrew Union College (HUC), in Cincinnati, Ohio, to begin his rabbinical training.
After a two-year interim during World War I, when he served in the American military, Marcus returned to graduate studies in Cincinnati. After receiving rabbinical ordination in 1920, Marcus was appointed to the faculy of HUC, where he began teaching biblical history. In 1922, Marcus travelled to Berlin to study Jewish history with Ismar Elbogen, who awarded Marcus a Ph.D. in 1925. In that year, Marcus married Antoinette Brody in Paris. After briefly studying at Hebrew University in Jerusalem in 1926, Marcus returned to Cincinnati, where he lectured at HUC consistently until 1995. In 1959, he was named the Adolph S. Ochs Professor of American Jewish History. In 1965, he was appointed to HUC's Milton and Hattie Kutz Distinguished Service Chair in American Jewish History.
Marcus devoted most of his post-World War II historical career to American Jewish history and founded the American Jewish Archives in Cincinnati in 1947 on the campus of HUC. He is, perhaps, best known for his work on Medieval European Jewish history, The Jew in the Medieval World: A Source Book: 315-1791, first published in 1938.