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ITER, officially the International Thermonuclear Experimental Reactor, is an international collaborative project to demonstrate the feasibility of fusion power for peaceful use. It aims to show that a controlled fusion reaction can produce more energy than is required to sustain it, and that the technology can be scaled toward practical fusion power plants. The facility is a tokamak—a magnetic confinement device that uses superconducting magnets to contain a hot plasma of deuterium and tritium at temperatures around 150 million degrees Celsius.

The project is being built at the Cadarache site in Saint-Paul-lez-Durance, France. It is governed by the ITER Organization, with seven member parties: the European Union (host), China, India, Japan, Korea, Russia, and the United States. Each member has a Domestic Agency responsible for supplying components and expertise.

ITER aims to achieve a fusion power gain (Q) of at least 10, meaning it would produce at least 500 MW of fusion power with a total input heating power around 50 MW, for durations of several hundred seconds. The experiment will test integrated technologies essential for a future fusion power plant, including superconducting magnets, a large cryogenic system, remote handling, tritium breeding considerations, and advanced plasma control.

Construction began in the 2010s, and the project has experienced delays and budget revisions. As of the mid-2020s, first plasma is targeted in the mid to late 2020s, with full deuterium-tritium operation following in subsequent years. ITER is intended to pave the way for DEMO, a demonstrator fusion power plant, and to advance fusion-energy development globally.