TRIGA (Training, Research, Isotopes, General Atomics) is a class of nuclear research reactor designed and manufactured by General Atomics. The design team for TRIGA, which included Edward Teller, was led by the physicist Freeman Dyson. Multiple variants of the reactor have been built, with a total of 66 having been installed across 24 different countries. The reactors have constant thermal power outputs ranging from 0.1-16 MW and can be pulsed to 22,000 MW.

TRIGA is a swimming pool reactor that can be installed without a containment building, and is designed for research and testing use by scientific institutions and universities for purposes such as undergraduate and graduate education, private commercial research, non-destructive testing and isotope production.

The TRIGA reactor uses uranium zirconium hydride (UZrH) fuel, which has a large, prompt negative fuel temperature coefficient of reactivity, meaning that as the temperature of the core increases, the reactivity rapidly decreases. Because of this unique feature, it has been safely pulsed at a power of up to 22,000 megawatts. The hydrogen in the fuel is bound in the uranium zirconium hydride crystal structure with a vibrational energy of 0.14eV. These levels fill when the fuel is hot, and transfer energy to thermal neutrons making them more energetic and, therefore, less likely to cause a fission. TRIGA was originally designed to be fueled with highly enriched uranium, but in 1978 the US Department of Energy launched its Reduced Enrichment for Research Test Reactors program, which promoted reactor conversion to low-enriched uranium fuel.

The concept of the TRIGA reactor was invented in the summer of 1956 when the president of the newly founded company General Atomics, Frederic de Hoffmann, invited a group 30–40 nuclear scientists (many of them alumni of the Manhattan Project) to consider what types of reactors would be commercially promising. The exploratory work focused on three concepts: a "Ship Reactor" (to propel merchant ships), a "Test Reactor" (for experimentally irradiating parts of nuclear power reactors), and a "Safe Reactor" (which eventually became TRIGA). During June-September the scientists worked out designs for the three reactors, and at the end of the summer de Hoffmann chose TRIGA for commercial development.

The "Safe reactor" working group was led by Edward Teller, who proposed that General Atomics should break into the market by developing a reactor that was much safer than its competitors. In Teller's words the reactor should be safe "even in the hands of a young graduate student", or in the words of Freeman Dyson "even in the hands of an idiot clever enough to by-pass the entire control system and blow out the control rods with dynamite." The working group comprised ten physicists, chemists, and engineers, including Freeman Dyson (who worked on the mathematical theory) and Massoud T. Simnad (who developed the fuel chemistry). At the end of the summer the preliminary design was handed over to a smaller team of General Atomics scientists, Ted Taylor, Stan Koutz, and Andrew McReynolds, for detailed development. The prototype for the TRIGA nuclear reactor (TRIGA Mark I) was commissioned on 3 May 1958 on the General Atomics campus in San Diego and operated until shut down in 1997. It has been designated as a nuclear historic landmark by the American Nuclear Society.

Mark II, Mark III, and other variants of the TRIGA design have subsequently been produced, and a total of 33 TRIGA reactors have been installed at locations across the United States. Those that remain operational continue to be upgraded or modernized. A further 33 reactors have been installed in other countries. Many of these installations were prompted by US President Eisenhower's 1953 Atoms for Peace policy, which sought to extend access to nuclear physics to countries in the American sphere of influence. Consequently, TRIGA reactors can be found in a total of 24 countries, including Austria, Bangladesh, Brazil, Congo, Colombia, United Kingdom, Finland, Germany, Taiwan, Japan, South Korea, Italy, Indonesia, Malaysia, Mexico, Morocco, Philippines, Puerto Rico, Romania, Slovenia, Thailand, Turkey, and Vietnam.

TRIGA International, a joint venture between General Atomics and CERCA [fr]—then a subsidiary of AREVA of France—was established in 1996. Since then, all TRIGA fuel assemblies have been manufactured at CERCA's plant in Romans-sur-Isère, France.

Some of the main competitors to General Atomics in the supply of research reactors are Korea Atomic Energy Research Institute (KAERI) of South Korea and INVAP of Argentina.