Bernard Bailyn (September 10, 1922 – August 7, 2020) was an American historian, author, and academic specializing in U.S. Colonial and Revolutionary-era History. He was a professor at Harvard University from 1953. Bailyn won the Pulitzer Prize for History twice (in 1968 and 1987). In 1998 the National Endowment for the Humanities selected him for the Jefferson Lecture. He was a recipient of the 2010 National Humanities Medal.

He specialized in American colonial and revolutionary-era history, looking at merchants, demographic trends, Loyalists, international links across the Atlantic, and especially the political ideas that motivated the Patriots. He was best known for studies of republicanism and Atlantic history that transformed the scholarship in those fields. He was elected a Fellow of the American Academy of Arts and Sciences in 1963 and a member of the American Philosophical Society in 1971.

Bailyn was born in Hartford, Connecticut, in 1922, the son of Esther (Schloss) and Charles Manuel Bailyn. His family was Jewish. Bailyn earned his bachelor's degree from Williams College in 1945 and in 1953 earned his Ph.D. from Harvard University. He was associated with Harvard for the rest of his life. As a graduate student at Harvard, he studied under Perry Miller, Samuel Eliot Morison, and Oscar Handlin. He was made a full professor in 1961, and professor emeritus in 1993.

Bernard Bailyn was the author of The Ideological Origins of the American Revolution (1967), which was awarded the Pulitzer Prize for History in 1968. He was the editor of The Apologia of Robert Keayne (1965) and of the two-volume Debate on the Constitution (1993).

He co-authored The Great Republic (1977), an American history textbook, and was co-editor of The Intellectual Migration, Europe and America, 1930–1960 (1969), Law in American History (1972), The Press and the American Revolution (1980), and Strangers Within the Realm: Cultural Margins of the First British Empire (1991).

Bailyn's dissertation and first publications dealt with New England merchants. He argued that international commerce was an uncertain business, given the high risk of losses at sea in the very long turnaround times, which meant that information was often too old to be useful. Merchants reduced the uncertainty by pooling their resources, especially with marriages to other merchant families, and placing their kinfolk as trusted agents in London and other foreign ports.

International commerce became a chief means of growing rich in colonial Massachusetts. However, there was an ongoing tension between the entrepreneurial spirit on the one hand and traditional Puritan culture on the other. The world of merchants became an engine of social change, undermining the isolationism, scholasticism, and religious zeal of the Puritan leadership. Bailyn pointed the younger generation of historians away from Puritan theology and toward broader social and economic forces. Bailyn expanded his research to the social structure of Virginia, showing how its leadership class was transformed in the 1660s. Like Edmund Morgan at Brown University and Yale, Bailyn emphasized the multiple roles of the family in the colonial social system.

In The Ideological Origins of the American Revolution Bailyn analyzed pre-Revolutionary political pamphlets to show that colonists believed the British intended to establish a tyrannical state that would abridge the historical British rights. He thus argued that the Revolutionary rhetoric of liberty and freedom was not simply propagandistic but rather central to their understanding of the situation. This evidence was used to displace Charles A. Beard's theory, then the dominant understanding of the American Revolution, that the American Revolution was primarily a matter of class warfare and that the rhetoric of liberty was meaningless. Bailyn maintained that ideology was ingrained in the revolutionaries, an attitude he said exemplified the "transforming radicalism of the American Revolution."