The Port Arthur massacre took place during the First Sino-Japanese War from 21 November 1894 for three days, when advance elements of the First Division of the Japanese Second Army under the command of General Yamaji Motoharu (1841–1897) killed somewhere between 2,600 civilians and 20,000 including Chinese soldiers in the Chinese coastal city of Port Arthur (now Lüshunkou District of Dalian, Liaoning).

As part of its wartime strategy during the First Sino-Japanese War, Japan had advanced through Korea, engaging Chinese troops at Asan near Seoul and then Pyongyang in September 1894, winning decisive victories on both occasions. Following the victory at Pyongyang the Japanese Second Army under Marshal Oyama Iwao (1842–1916) moved northward towards Manchuria, the plan being to seize Port Arthur, headquarters to China's Beiyang Fleet and a highly fortified city that dominated the sea passage from Korea to northeast China. In September the Japanese Navy heavily damaged the Beiyang Fleet at the Battle of the Yalu River, though the Chinese troopships were successful in landing their troops not far from the Sino-Korean border. With the Beiyang Fleet defeated, the Japanese Navy began a siege of Port Arthur while the Japanese Second Army advanced on the city through Manchuria and the Japanese First Army crossed the Yalu River to form another advance by land. After a series of battles on the Liaodong Peninsula the First Division of the Second Army, led by General Yamaji, drew up around Port Arthur in late November. On 18 November 1894, the Japanese movement down the peninsula was temporarily frustrated and returned to find that their abandoned wounded troops had been severely mutilated, with hands and feet cut off. Others had been burned alive. The city was evacuated with residents fleeing westward by land or sea into China. The Qing government placed bounties on prisoners of war, or their heads or other body parts; during the Sino-Japanese War the bounty was 50 taels. Some Chinese soldiers had mutilated several Japanese soldiers' dead bodies and displayed them at the entrance of the city, infuriating the Japanese. Several vowed revenge, including Lieutenant Kijiro Nanbu. After only token resistance, the city fell to Japanese troops late on the morning of 21 November. What followed was a massacre of the remaining inhabitants of Port Arthur by the Japanese troops.

The scale and nature of the killing continues to be debated. Japanese participants reported mountains of corpses, yet the number of dead was difficult to calculate. According to a scouting report sent to Viceroy Li Hongzhang by the local official Liu Hanfang soon after the massacre, 2,600~2,700 civilians were slaughtered within the city. However, many more were slaughtered in the hills surrounding the city but they had no reliable count. In 1948, the Chinese Communist Party built a cemetery and marked the total deaths to be 20,000, which include soldiers killed in action and fleeing soldiers disguised as civilians. The 20,000 figure became the orthodox figure in Chinese communist sources.