The Nanking Massacre, commonly known as the Rape of Nanking, was an infamous genocidal war crime committed by the Japanese military in Nanjing, then capital of the Republic of China, after it fell to the Imperial Japanese Army on December 13, 1937. The duration of the massacre is not clearly defined, although the violence lasted well into the next six weeks, until early February 1938.

During the occupation of Nanking, the Japanese army committed numerous atrocities, such as rape, looting, arson and the execution of prisoners of war and civilians. Although the executions began under the pretext of eliminating Chinese soldiers disguised as civilians, it is claimed that a large number of innocent men were intentionally identified as enemy combatants and executed as the massacre gathered momentum. A large number of women and children were also killed, as rape and murder became more widespread.

According to the International Military Tribunal for the Far East, estimates made at a later date indicate that the total number of civilians and prisoners of war murdered in Nanking and its vicinity during the first six weeks of the Japanese occupation was over 200,000. That these estimates are not exaggerated is borne out by the fact that burial societies and other organizations counted more than 155,000 buried bodies. Most were bound with their hands tied behind their backs. These figures do not take into account those persons whose bodies were destroyed by burning, by throwing them into the Yangtze River, or otherwise disposed of by the Japanese." The extent of the atrocities is debated between China and Japan, with numbers ranging from some Japanese claims of several hundred, to the Chinese claim of a non-combatant death toll of 300,000 . A number of Japanese researchers consider 100,000 – 200,000 to be an approximate value. Other nations usually believe the death toll to be between 150,000–300,000. This number was first promulgated in January of 1938 by Harold Timperly, a journalist in China during the Japanese invasion, based on reports from contemporary eyewitnesses. Other sources, including Iris Chang's The Rape of Nanking, also promote 300,000 as the death toll. In addition, on December 12, 2007, newly declassified U.S. government documents revealed an additional toll of around 500,000 in the area surrounding Nanking before it was occupied.

In addition to the number of victims, some Japanese critics have even disputed whether the atrocity ever happened. While the Japanese government has acknowledged the incident did occur , some Japanese nationalists have argued, partly using the Imperial Japanese Army's claims at the International Military Tribunal for the Far East, that the death toll was military in nature and that no such civilian atrocities ever occurred. This claim has been criticised by various figures, citing statements of non-Chinese at the Tribunal, other eyewitnesses and by photographic and archaeological evidence that civilian deaths did occur.

Condemnation of the massacre is a major focal point of Chinese nationalism. In Japan, however, public opinion over the severity of the massacre remains widely divided — this is evidenced by the fact that whereas some Japanese commentators refer to it as the 'Nanking massacre', others use the more ambivalent term 'Nanking Incident'. However, this term can also refer to a separate Nanjing Incident that occurred during the 1927 Nationalist seizure of the city as a part of the Northern Expedition, in which foreigners in the city were attacked. The 1937 massacre and the extent of its coverage in school textbooks continues to be a point of contention and controversy in Sino-Japanese relations.