Arthur John Brereton Marwick FRHistS (29 February 1936 – 27 September 2006) was a British social historian, who served for many years as Professor of History at the Open University. His research interests lay primarily in the history of Britain in the twentieth century, and the relationship between war and social change. He is probably best known, however, for his more theoretical book The Nature of History (1970; revised editions 1981 and 1989), and its greatly reworked and expanded version The New Nature of History (2001). In the latter work he defended an empirical and source-based approach towards the writing of history, and argued against the turn towards postmodernism. He believed firmly that history was "of central importance to society".

Marwick was born on 29 February 1936 in Edinburgh, the younger son of William Hutton Marwick (1894–1982), an economic historian, and his wife, Maeve Cluna, née Brereton. His parents were Quakers. He was educated at George Heriot's School, Edinburgh, and at the University of Edinburgh, before undertaking postgraduate research at Balliol College, Oxford. His Oxford thesis, on the Independent Labour Party, was awarded a BLitt, but not a doctorate.

Marwick worked as an assistant lecturer in history at the University of Aberdeen in 1959–60, before becoming a lecturer in history at the University of Edinburgh in 1960. In 1969 he was appointed the first professor of history at the newly established Open University, where he remained until his retirement in 2001. From 1978 until 1984 he served as dean and director of studies in arts at the Open University. He also held brief visiting professorships at the State University of New York at Buffalo, Stanford University, Rhodes College, the University of Perugia and the École des Hautes Études en Sciences Sociales in Paris.

From 1995 to 1998 he served as co-editor of the Journal of Contemporary History.

Marwick was a left-wing social and cultural historian, but critical of Marxism and other approaches to history that he believed stressed the importance of metanarrative over archival research. He was also a critic of postmodernism.

Particularly in his books The Nature of History (1970), and its expanded version The New Nature of History (2001), Marwick analysed at length different types of historical evidence, distinguishing between "facts" and sources, primary and secondary sources, and different varieties of primary source. He also distinguished between "witting" and "unwitting" testimony; that is, between the overt and intentional message of a document or source, and the unintentional evidence that it also contains.

One of Marwick's most influential books, The Deluge (1965), dealt with the transformations in British society brought about by the First World War. Its main thesis was that the war had brought about positive and lasting social changes (in the rôle of women, in the acceptability of state intervention for social reasons, and so on). Despite its terrible tragedies, Marwick believed that the sum result of the war was that Britain was a better place to live in the 1920s than in the period before the war.

He analysed the social changes that result from total war in terms of four different "modes", or dimensions: