The Great Terror: Stalin's Purge of the Thirties is a book by British historian Robert Conquest which was published in 1968. It gave rise to an alternate title of the period in Soviet history known as the Great Purge. Conquest's title was also an evocative allusion to the period that was called the Reign of Terror during the French Revolution (French: la Terreur and from June to July 1794 la Grande Terreur, "the Great Terror"). A revised version of the book, called The Great Terror: A Reassessment, was printed in 1990 after Conquest was able to amend the text, having consulted the opened Soviet archives. The book was funded and widely disseminated by Information Research Department, who also published Orwell's list collected by Conquest's secretary Celia Kirwan.

One of the first books by a Western writer to discuss the Great Purge in the Soviet Union, it was based mainly on information which had been made public, either officially or by individuals, during the Khrushchev Thaw in the period 1956–1964, and on an analysis of official documents such as the Soviet census. It also drew on accounts by Russian and Ukrainian émigrés and exiles dating back to the 1930s. The book was well received in the popular press but its estimates started a debate among historians. Conquest defended his higher estimates of 20 million, which are supported by some historians and other authors in the popular press, while other historians said that even his reassessments were still too high and are considerably less than originally thought.