放射性同位素（英语：radionuclide，或radioactive nuclide），一种具有放射性的核素。是一种原子核不稳定的原子，每个原子也有很多同位素，每组同位素的原子序虽然是相同，但是却有着不同的原子量，如果这原子是有放射性的话，它会被称为物理放射性核种或放射性同位素。放射性同位素会进行放射性衰变，从而放射出伽玛射线，和次原子粒子。

化学家和生物学家都把放射性同位素的技术应用在我们的食品、水和身体健康等事项上。不过他们也察觉到危险性，因而制订使用的安全守则。有些放射性同位素是天然存在的，有些则是人工制造的，称为人造放射性同位素。

A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is an atom that has excess nuclear energy, making it unstable. This excess energy can either create and emit, from the nucleus, new radiation (gamma radiation) or a new particle (alpha particle or beta particle), or transfer this excess energy to one of its electrons, causing it to be ejected (conversion electron). During this process, the radionuclide is said to undergo radioactive decay. These emissions constitute ionizing radiation. The unstable nucleus is more stable following the emission, but will sometimes undergo further decay. Radioactive decay is a random process at the level of single atoms: it is impossible to predict when one particular atom will decay. However, for a collection of atoms of a single element the decay rate, and thus the half-life (t1/2) for that collection can be calculated from their measured decay constants. The duration of the half-lives of radioactive atoms have no known limits; the time range is over 55 orders of magnitude.