World War II saw the largest scale of war crimes and crimes against humanity ever committed in an armed conflict, mostly against civilians and specific groups (e.g. Jews, homosexuals, people who are mentally ill or disabled) and POWs. The war also saw the indiscriminate mass rape of captured women, carpet bombing of civilian targets and use of starvation as weapon of war.

Most of these crimes were carried out by the Axis powers who constantly violated the rules of war and the Geneva Convention on Prisoners of War, mostly by Nazi Germany and the Empire of Japan. Dutch historian Pieter Lagrou [nl] observed that "forced labor carried out in murderous circumstances by Allied soldiers and civilians in Japanese hands", alongside the murder of millions of Soviet POWs by the Germans, "are among the most infamous crimes of the Second World War".

However the decision by the United States to drop nuclear bombs on Hiroshima and Nagasaki is still debated to this day on whether it could amount to war crimes or crimes against humanity.

This is a list of war crimes committed during World War II.

The Axis powers (Nazi Germany, Fascist Italy, and the Empire of Japan) were some of the most systematic perpetrators of war crimes in modern history. The factors which contributed to Axis war crimes included Nazi racial theories, the desire for "living space" which was used as a justification for the eradication of native populations, and militaristic indoctrination that encouraged the terrorization of conquered peoples and prisoners of war. The Holocaust, the German attack on the Soviet Union and the German occupation of much of Europe, the Japanese invasion and occupation of Manchuria, the Japanese invasion of China and the Japanese occupation of the Philippines all contributed to well over half of all of the civilian deaths in World War II as well as the conflicts that led up to the war. Even before post-war revelations of atrocities, Axis military forces were notorious for their brutal treatment of captured combatants.

According to the Nuremberg Trials, there were four major categories of crimes alleged against the German political leadership, the ruling party NSDAP, the military high command, the paramilitary SS, the security services, the civil occupation authorities, as well as individual government officials (including members of the civil service or the diplomatic corps), soldiers or members of paramilitary formations, industrialists, bankers and media proprietors, each with individual events that made up the major charges. The crime of genocide was later raised to a separate, fifth category.

At least 10 million, and perhaps over 20 million perished directly and indirectly due to the commission of crimes against humanity and war crimes by Hitler's regime, of which the Holocaust lives on in particular infamy, for its particularly cruel nature and scope, and the industrialised nature of the genocide of Jewish citizens of states invaded or controlled by the Nazi regime. At least 5.9 million Jews were murdered by the Nazis, or 66 to 78% of Europe's Jewish population, although a complete count may never be known. Though much of Continental Europe suffered under the Nazi occupation, Poland, in particular, was the state most devastated by these crimes, with 90% of its Jews as well as many ethnic Poles slaughtered by the Nazis and their affiliates. After the war, from 1945 to 1949, the Nazi regime was put on trial in two tribunals in Nuremberg, Germany by the victorious Allied powers.

The first tribunal indicted 24 major Nazi war criminals, and resulted in 19 convictions (of which 12 led to death sentences) and 3 acquittals, 2 of the accused died before a verdict was rendered, at least one of which by killing himself with cyanide. The second tribunal indicted 185 members of the military, economic, and political leadership of Nazi Germany, of which 142 were convicted and 35 were acquitted. In subsequent decades, approximately 20 additional war criminals who escaped capture in the immediate aftermath of World War II were tried in West Germany and Israel. In Germany and many other European nations, the Nazi Party and denial of the Holocaust is outlawed.^{[citation needed]}