Gamma rays (denoted as γ) are electromagnetic radiation of high energy. They are produced by sub-atomic particle interactions, such as electron-positron annihilation, neutral pion decay, radioactive decay, fusion, fission or inverse Compton scattering in astrophysical processes. Gamma rays typically have frequencies above 1019 Hz and therefore energies above 100 keV and wavelength less than 10 picometers, often smaller than an atom. Gamma radioactive decay photons commonly have energies of a few hundred KeV, and are almost always less than 10 MeV in energy.

Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900, while studying radiation emitted from radium. Alpha and beta "rays" had already been separated and named by the work of Ernest Rutherford in 1899, and in 1903 Rutherford named Villard's distinct new radiation "gamma rays."

Hard X-rays produced for by linear accelerators ("linacs") and astrophysical processes often have higher energy than gamma rays produced by radioactive gamma decay. In fact, one of the most common gamma-ray emitting isotopes used in nuclear medicine, technetium-99m produces gamma radiation of about the same energy (140 kev) as produced by a diagnostic X-ray machine, and significantly lower energy than the therapeutic treatment X-rays produced by linac machines in cancer radiotherapy.

In the past, distinction between the X-rays and gamma rays was arbitrarily based on energy (or equivalently frequency or wavelength), but because of the wide overlap and increasing use of megavoltage X-ray sources, now the two types of radiation are usually defined by their origin: X-rays are emitted by electrons outside the nucleus (and when produced by therapeutic linacs are often simply called "photons"), while gamma rays are specifically emitted by the nucleus (that is, produced by gamma decay). In theory, there is no lower limit to the energy of such photons, and thus "ultraviolet gamma rays" have been postulated.

In certain fields such as astronomy, gamma rays and X-rays are still sometimes defined by energy, as the processes which produce them may be uncertain.

As a form of ionizing radiation, gamma rays can cause serious damage when absorbed by living tissue, and they are therefore a health hazard.

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