American historian noted for his scholarly works on the Protestant Reformation.

The son of a prominent Presbyterian clergyman, Smith earned his Ph.D. at Columbia University (1907). He was subsequently a fellow in history at Amherst College (Amherst, Mass.) until 1914. He lectured in history at Harvard University (1919–20) and was made professor of history at Cornell University (1922), where he remained until his death.

His dissertation, Luther's Table Talk (1907), was enlarged and published as a full biography, The Life and Letters of Martin Luther (1911), in which he saw the Reformation as the most significant period of change in modern thought and Luther as its leader. In 1920 he published his great work, The Age of the Reformation, a comprehensive survey of the economic, intellectual, and social context of Protestantism in the 16th century. His book Erasmus: A Study of His Life, Ideals, and Place in History (1923) probed the relationship between the Renaissance and the Reformation and characterized Erasmus as the Reformation's proponent of intellectual freedom. Of his projected four-volume History of Modern Culture, he completed two volumes entitled The Great Renewal, 1543–1687 (1930) and The Enlightenment, 1687–1776 (1934).