A magnetohydrodynamic generator (MHD generator) is a magnetohydrodynamic converter that transforms thermal energy and kinetic energy directly into electricity. An MHD generator, like a conventional generator, relies on moving a conductor through a magnetic field to generate electric current. The MHD generator uses hot conductive ionized gas (a plasma) as the moving conductor. The mechanical dynamo, in contrast, uses the motion of mechanical devices to accomplish this.

MHD generators are different from traditional electric generators in that they operate without moving parts (e.g. no turbines), so there is no limit on the upper temperature at which they can operate. They have the highest known theoretical thermodynamic efficiency of any electrical generation method. MHD has been developed for use in combined cycle power plants to increase the efficiency of electric generation, especially when burning coal or natural gas. The hot exhaust gas from an MHD generator can heat the boilers of a steam power plant, increasing overall efficiency.

Practical MHD generators have been developed for fossil fuels, but these were overtaken by less expensive combined cycles in which the exhaust of a gas turbine or molten carbonate fuel cell heats steam to power a steam turbine.

MHD dynamos are the complement of MHD accelerators, which have been applied to pump liquid metals, seawater, and plasmas.

Natural MHD dynamos are an active area of research in plasma physics and are of great interest to the geophysics and astrophysics communities since the magnetic fields of the Earth and Sun are produced by these natural dynamos.

In a conventional thermal power plant, like a coal-fired power station or nuclear power plant, the energy created by the chemical or nuclear reactions is absorbed in a working fluid, usually water. In a coal plant, for instance, the coal burns in an open chamber which is surrounded by tubes carrying water. The heat from the combustion is absorbed by the water which boils into steam. The steam is then sent into a steam turbine which extracts energy from the steam by turning it into rotational motion. The steam is slowed and cooled as it passes through the turbine. The rotational motion then turns an electrical generator.

The efficiency of this overall cycle, known as the Rankine cycle, is a function of the temperature difference between the inlet to the boiler and the outlet to the turbine. The maximum temperature at the turbine is a function of the energy source; and the minimum temperature at the inlet is a function of the surrounding environment's ability to absorb waste heat. For many practical reasons, coal plants generally extract about 35% of the heat energy from the coal, the rest is ultimately dumped into the cooling system or escapes through other losses.

MHD generators can extract more energy from the fuel source than turbine-generator systems. They do this by skipping the step where the heat is transferred to another working fluid. Instead, they use the hot exhaust directly as the working fluid. In the case of a coal plant, the exhaust is directed through a nozzle that increases its velocity, essentially a rocket nozzle, and then directs it through a magnetic system that directly generates electricity. In a conventional generator, rotating magnets move past a material filled with nearly-free electrons, typically copper wire (or vice versa depending on the design). In the MHD system the electrons in the exhaust gas move past a stationary magnet. Ultimately the effect is the same, the working fluid is slowed down and cools as its kinetic energy is transferred to electrons, and is thereby converted to electrical power.