A light water reactor or LWR is a thermal nuclear reactor that uses ordinary water, also called light water, as its neutron moderator. This differentiates it from a heavy water reactor, which uses heavy water as a neutron moderator. In practice all LWRs are also water cooled.

Many other reactors are also (light) water cooled, notably the RBMK and some military plutonium production reactors. These are not regarded as LWRs, as they are moderated by graphite, and as a result their nuclear characteristics are very different.

The most common LWRs are pressurized water reactors and boiling water reactors.

The light-water reactor uses uranium 235 as a fuel, enriched to approximately 3 percent. Although this is its major fuel, the uranium 238 atoms also contribute to the fission process by converting to plutonium 239 - about one-half of which is consumed with the reactor. Light-water reactors are generally refueled every 12 to 18 months. At which time, about 25 percent of the fuel is replaced.

The U.S.–North Korean Agreed Framework as well as the Six-party talks involved the supply of LWRs to North Korea.

Nuclear reactors | Nuclear power reactor types | Réacteur à eau légère | Leichtwasserreaktor | 軽水炉 | Kevytvesireaktori | Легководный реактор | Lättvattenreaktor