World War III or the Third World War, often abbreviated as WWIII or WW3, are names given to a hypothetical worldwide large-scale military conflict subsequent to World War I and World War II. The term has been in use since at least as early as 1941. Some apply it loosely to limited or more minor conflicts such as the Cold War or the war on terror. In contrast, others assume that such a conflict would surpass prior world wars in both scope and destructive impact.

Due to the development of nuclear weapons in the Manhattan Project, which were used in the atomic bombings of Hiroshima and Nagasaki near the end of World War II, and their subsequent acquisition and deployment by many countries afterward, the potential risk of a nuclear apocalypse causing widespread destruction of Earth's civilization and life is a common theme in speculations about a third world war. Another primary concern is that biological warfare could cause many casualties. It could happen intentionally or inadvertently, by an accidental release of a biological agent, the unexpected mutation of an agent, or its adaptation to other species after use. Large-scale apocalyptic events like these, caused by advanced technology used for destruction, could render most of Earth's surface uninhabitable.

Before the beginning of World War II in 1939, World War I (1914–1918) was believed to have been "the war to end [all] wars". It was popularly believed that never again could there possibly be a global conflict of such magnitude. During the interwar period, World War I was typically referred to simply as "The Great War". The outbreak of World War II disproved the hope that humanity might have "outgrown" the need for widespread global wars.

With the advent of the Cold War in 1945 and with the spread of nuclear weapons technology to the Soviet Union, the possibility of a third global conflict increased. During the Cold War years, the possibility of a third world war was anticipated and planned for by military and civil authorities in many countries. Scenarios ranged from conventional warfare to limited or total nuclear warfare. At the height of the Cold War, the doctrine of mutually assured destruction (MAD), which determined that an all-out nuclear confrontation would destroy all of the states involved in the conflict, developed. The potential for the absolute destruction of the human species may have contributed to both American and Soviet leaders avoiding such a scenario.

The various global military conflicts that have occurred since the start of the 21st century, most recently the Russian invasion of Ukraine, alongside rising tensions between the United States and China, have been perceived as potential flashpoints or triggers for a third world war.