A light water reactor or LWR is a thermal nuclear reactor that uses ordinary water (as opposed to heavy water) as its neutron moderator. In practice all LWRs are also water cooled.

However, many other reactors are also water cooled, notably the RBMK and some military plutonium production reactors. These are not regarded as LWRs as their nuclear characteristics are very different.

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