A molten-salt reactor (MSR) is a class of nuclear fission reactor in which the primary nuclear reactor coolant and/or the fuel is a mixture of molten salt with a fissile material.

Two research MSRs operated in the United States in the mid-20th century. The 1950s Aircraft Reactor Experiment (ARE) was primarily motivated by the technology's compact size, while the 1960s Molten-Salt Reactor Experiment (MSRE) aimed to demonstrate a nuclear power plant using a thorium fuel cycle in a breeder reactor.

Increased research into Generation IV reactor designs renewed interest in the 21st century with multiple nations starting projects. On October 11, 2023, China's TMSR-LF1 reached criticality, and subsequently achieved full power operation, as well as Thorium breeding.

MSRs eliminate the nuclear meltdown scenario present in water-cooled reactors because the fuel mixture is kept in a molten state. The fuel mixture is designed to drain without pumping from the core to a containment vessel in emergency scenarios, where the fuel solidifies, quenching the reaction. In addition, hydrogen evolution does not occur. This eliminates the risk of hydrogen explosions (as in the Fukushima nuclear disaster). They operate at or close to atmospheric pressure, rather than the 75–150 times atmospheric pressure of a typical light-water reactor (LWR). This reduces the need and cost for reactor pressure vessels. The gaseous fission products (Xe and Kr) have little solubility in the fuel salt, and can be safely captured as they bubble out of the fuel, rather than increasing the pressure inside the fuel tubes, as happens in conventional reactors. MSRs can be refueled while operating (essentially online-nuclear reprocessing) while conventional reactors shut down for refueling (notable exceptions include pressure tube reactors like the heavy water CANDU or the Atucha-class PHWRs, light water cooled graphite moderated RBMK, and British-built gas-cooled reactors such as Magnox, AGR). MSR operating temperatures are around 700 °C (1,292 °F), significantly higher than traditional LWRs at around 300 °C (572 °F). This increases electricity-generation efficiency and process-heat opportunities.

Relevant design challenges include the corrosivity of hot salts and the changing chemical composition of the salt as it is transmuted by the neutron flux.

MSRs, especially those with fuel in the molten salt, offer lower operating pressures, and higher temperatures. In this respect an MSR is more similar to a liquid metal cooled reactor than to a conventional light water cooled reactor. MSR designs are often breeding reactors with a closed fuel cycle—as opposed to the once-through fuel currently used in conventional nuclear power generators.

MSRs exploit a negative temperature coefficient of reactivity and a large allowable temperature rise to prevent criticality accidents. For designs with the fuel in the salt, the salt thermally expands immediately with power excursions. In conventional reactors the negative reactivity is delayed since the heat from the fuel must be transferred to the moderator. An additional method is to place a separate, passively cooled container below the reactor. Fuel drains into the container during malfunctions or maintenance, which stops the reaction.

The temperatures of some designs are high enough to produce process heat, which led them to be included on the GEN-IV roadmap.