A generation 2 nuclear reactor is a design classification for a nuclear reactor, and refers to the class of commercial reactors built up to the end of the 1990s. Prototypical generation II reactors include the PWR, CANDU, BWR, AGR, and VVER.

These are contrasted to generation I reactors, which refer to the early prototype and power reactors, such as Shippingport, Magnox, Fermi 1, and Dresden. The nomenclature for reactor designs, describing four 'generations', was proposed by the US Department of Energy when it introduced the concept of generation IV reactors.

The designation generation II+ reactor is sometimes used for modernised generation II designs built post-2000, such as the Chinese CPR-1000, in competition with more expensive generation III reactor designs. Typically the modernisation includes improved safety systems and a 60 year design life.

Generation II reactor designs generally had an original design life of 30 or 40 years. However many generation II reactor are being life-extended to 50 or 60 years, and a second life-extension to 80 years may also be economic in many cases.