Transmutation is the conversion of one object into another. It is most commonly used in reference to extremely basic objects, for example turning lead to gold or gold to pure energy, but can be used to refer to more complex objects as well, as in transmogrification.

Transmutation of actinides such as the isotopes of plutonium, neptunium, americium, and curium has the potential to contribute to solving the problems posed by the management of radioactive waste, by reducing the proportion of long-lived isotopes it contains. When irradiated with neutrons in a nuclear reactor, these isotopes can be made to undergo nuclear fission, destroying the original actinide isotope and producing a spectrum of radioactive and nonradioactive fission products. Isotopes of plutonium and other actinides tend to be long-lived with half-lifes of many thousands of years, whereas radioactive fission products tend to be shorter-lived (most with half-lifes of 30 years or less). From a waste management viewpoint, transmutation of actinides eliminates a long-term radioactive hazard while producing a shorter-term radioactive hazard instead.

Transmutation of chemical elements occurs through nuclear reactions. In alchemy, it is believed that such transformations can be accomplished in table-top experiments, but this is not accepted science. Some researchers say they have found evidence of transmutation of elements in biological processes (see Kervran). Some (Yasuhiro Iwamura) have reported transmutation via the transport of deuterium gas through a palladium wall into vacuum. Others have reported cold fusion in electrolytic cells.

The term dates back to the search for the philosopher's stone. It was applied consciously to modern physics first by Frederick Soddy when he, along with Ernest Rutherford, discovered that radioactive thorium was converting itself into radium in 1901. At the moment of realization, Soddy later recalled, he shouted out: "Rutherford, this is transmutation!" Rutherford snapped back, "For Mike's sake, Soddy, don't call it transmutation. They'll have our heads off as alchemists."

See also: transubstantiation