World War III, also known as the Third World War, is a hypothetical future global conflict subsequent to World War I (1914–1918) and World War II (1939–1945). It is widely predicted that such a war would involve all of the great powers, like its two predecessors, and the use of nuclear weapons or other weapons of mass destruction, thereby surpassing all prior conflicts in scale, devastation, and loss of life.

World War III was initially synonymous with the escalation of the Cold War (1947–1991) into direct conflict between the US-led Western Bloc and Soviet-led Eastern Bloc. Since the United States' development and use of nuclear weapons in the atomic bombings of Hiroshima and Nagasaki at the end of World War II, the risk of a nuclear apocalypse causing widespread destruction and the potential collapse of modern civilization or human extinction has been central in speculation and fiction about World War III. The Soviet Union's development of nuclear weapons in 1949 spurred the nuclear arms race and was followed by several other countries.

Regional proxy wars including the Korean War (1950–1953), Vietnam War (1955–1975), and Soviet–Afghan War (1979–1989), while significant, did not lead to a full-scale global conflict. A global conflict was planned for by military and civil personnel around the world, with scenarios ranging from conventional warfare to limited or total nuclear warfare. The certainty of escalation from one stage to the next was extensively debated. For example, the Eisenhower administration promulgated a policy of massive retaliation with nuclear forces, to a minor conventional attack. After the Cuban Missile Crisis in 1962, which brought the US and Soviet Union to the brink of war, the strategic doctrine of mutually assured destruction, which held that a full-scale nuclear war would annihilate all parties, became widely accepted. At their 1985 summit, US and Soviet leaders first jointly stated "a nuclear war cannot be won and must never be fought". Advocates of deterrence theory hold that nuclear weapons prevent World War III–like great power conflict, while advocates of nuclear disarmament hold that their risks far outweigh this.

Since the end of the Cold War in 1991, speculation about World War III shifted toward emerging threats, including terrorism and cyberwarfare. Great-power competition was renewed between the United States, China, and Russia, sometimes termed a Second Cold War. Various conflicts, most significantly the Russian invasion of Ukraine (2022–present), the Middle Eastern crisis (2023–present), and rising tensions over the status of Taiwan, have been perceived as flashpoints for a third world war.

Time magazine was an early adopter, if not originator, of the term "World War III". The first usage appears in its 3 November 1941 issue (preceding the Japanese attack on Pearl Harbor on 7 December 1941) under its "National Affairs" section and entitled "World War III?" about Nazi refugee Hermann Rauschning, who had just arrived in the United States. In its 22 March 1943, issue under its "Foreign News" section, Time reused the same title "World War III?" about statements by then–US Vice President Henry A. Wallace: "We shall decide sometime in 1943 or 1944 ... whether to plant the seeds of World War III." Time continued to entitle with or mention in stories the term "World War III" for the rest of the decade and onwards: 1944, 1945, 1946 ("bacterial warfare"), 1947, and 1948. Time persists in using this term, for example, in a 2015 book review entitled "This Is What World War III Will Look Like".

Military strategists have used war games to prepare for various war scenarios and to determine the most appropriate strategies. War games were utilized for World War I and World War II.

British Prime Minister Winston Churchill was concerned that, with the enormous size of Soviet Red Army forces deployed in Central and Eastern Europe at the end of World War II and the perceived unreliability of the Soviet leader Joseph Stalin, there was a serious threat to Western Europe. In April–May 1945, the British Armed Forces developed Operation Unthinkable, thought to be the first scenario of the Third World War. Its primary goal was "to impose upon Russia the will of the United States and the British Empire". The plan was rejected by the British Chiefs of Staff Committee as militarily unfeasible.

Operation Dropshot was the 1950s United States contingency plan for a possible nuclear and conventional war with the Soviet Union in the Western European and Asian theaters. Although the scenario made use of nuclear weapons, they were not expected to play a decisive role.