The RBMK (Russian: Реактор большой мощности канальный, РБМК; reaktor bolshoy moshchnosti kanalnyy, "high-power channel-type reactor") is a class of graphite-moderated nuclear power reactor designed and built by the Soviet Union. It is somewhat like a boiling water reactor as water boils in the pressure tubes. It is one of two power reactor types to enter serial production in the Soviet Union during the 1970s, the other being the VVER reactor. The name refers to its design where instead of a large steel pressure vessel surrounding the entire core, the core is surrounded by a cylindrical annular steel tank inside a concrete vault and each fuel assembly is enclosed in an individual 8.4 cm (inner) diameter pipe (called a "technological channel"). The channels also contain the coolant, and are surrounded by graphite.

The RBMK is an early Generation II reactor and the oldest commercial reactor design still in wide operation. Certain aspects of the original RBMK reactor design had several shortcomings, such as the large positive void coefficient, the 'positive scram effect' of the control rods and instability at low power levels—which contributed to the 1986 Chernobyl disaster, in which an RBMK experienced an uncontrolled nuclear chain reaction, leading to a steam and hydrogen explosion, large fire, and subsequent core meltdown. Radioactive material was released over a large portion of northern and southern Europe—including Sweden, where evidence of the nuclear disaster was first registered outside of the Soviet Union, and before the Chernobyl accident was communicated by the Soviet Union to the rest of the world. The disaster prompted worldwide calls for the reactors to be completely decommissioned; however, there is still considerable reliance on RBMK facilities for power in Russia with the aggregate power of operational units at almost 7 GW of installed capacity. Most of the flaws in the design of RBMK-1000 reactors were corrected after the Chernobyl accident and a dozen reactors have since been operating without any serious incidents for over thirty years.

RBMK reactors may be classified as belonging to one of three distinct generations, according to when the particular reactor was built and brought online:

Beside RBMK, there are several other graphite-moderated reactors. A graphite-moderated Magnox reactor exists in North Korea at the Yongbyon Nuclear Scientific Research Center. While the gas cooled Magnox, AGR and pebble bed reactors (Such as the Dragon reactor at Winfrith) use graphite as moderators their use of gases (carbon dioxide for Magnox and AGR, while helium for Dragon) as heat transfer fluids causes them to have no void coefficient. 4 EGP-6 graphite water reactors which are a scaled down version of the RBMK were operating at the world's second northern most nuclear power plant i.e. the Bilibino Nuclear Power Plant. One EGP-6 was shut down permanently in 2020. The remaining three EGP-6 were shutdown permanently in December 2025.

Initially the service life of the RBMK reactor type was expected to be 30 years, but this may be extended to 45 years with mid-life refurbishments (such as fixing the issue of graphite stack deformation in the core), and eventually a 50-year lifetime was adopted for some units (Kursk 1-3 and 1-4, Leningrad 1-3 and 1-4, Smolensk 1-1, 1-2, 1-3). Efforts are underway to extend the license of all the units. In July 2024, Leningrad unit 3's license was extended from 2025 to 2030. In February 2026, the Russian nuclear regulator approved a five year life extension to Leningrad Unit 4 to operate until 2031. Today all the reactors are operated by Rosatom's subsidiary Rosenergoatom.

In 2026, it was reported that the RBMK units of the Kursk plant had undergone significant safety upgrades reducing risks by almost 100 times.

The RBMK was the culmination of the Soviet nuclear power program to produce a water-cooled power reactor with dual-use potential based on their graphite-moderated plutonium production military reactors. The first of these, Obninsk AM-1 ("Атом Мирный", Atom Mirny, Russian for "peaceful atom," analogous to the American Atoms for Peace) generated 5 MW of electricity from 30 MW thermal power, and supplied Obninsk from 1954 until 1959. Subsequent prototypes were the AMB-100 reactor and AMB-200 reactor both at Beloyarsk Nuclear Power Station.

The RBMK was a minimalist design that used regular (light) water for cooling and graphite for moderation, making it possible to use fuel with a lower enrichment (1.8% enriched uranium instead of considerably more expensive 4% enrichment). This allowed for an extraordinarily large and powerful reactor that could be built rapidly, largely out of parts fabricated on-site instead of by specialized factories. Because a containment building would have needed to be very large and expensive, doubling the cost of each unit, due to the large size of the RBMK, it was originally omitted from the design. It was argued by its designers that the RBMK's strategy of having each fuel assembly in its own channel with flowing cooling water was an acceptable alternative for containment.