A swimming pool reactor, also called an open pool reactor, is a type of nuclear reactor that has a core (consisting of the fuel elements and the control rods) immersed in an open pool usually of water.

The water acts as neutron moderator, cooling agent and radiation shield. The layer of water directly above the reactor core shields the radiation so completely that operators may work above the reactor safely. This design has two major advantages: the reactor is easily accessible and the whole primary cooling system, i.e. the pool water, is under normal pressure. This avoids the high temperatures and great pressures of nuclear power plants. Pool reactors are used as a source of neutrons and for training, and in rare instances for processing heat but not for electrical generation.