碘-131（Iodine-131），也称放射碘（Radioiodine），是碘的一种同位素。原子核内有78个中子，比碘的稳定性核素原子核的中子数多4个。碘-131是人工核裂变产物，正常情况下在自然界中不会存在，摄入人体后，会积聚在甲状腺处对人体造成危害。因此各国会针对核能电厂周边住民发放或储备碘片（碘化钾），核灾发生后遭受游离碘辐射暴露时，于4小时内服用可使碘在甲状腺里饱和，减少甲状腺对放射性碘－131的吸收。

Iodine-131 (I), is an important radioisotope of iodine discovered by Glenn Seaborg and John Livingood in 1938 at the University of California, Berkeley. It has a radioactive decay half-life of about eight days. It is associated with nuclear energy, medical diagnostic and treatment procedures, and natural gas production. It also plays a major role as a radioactive isotope present in nuclear fission products, and was a significant contributor to the health hazards from open-air atomic bomb testing in the 1950s, and from the Chernobyl disaster, as well as being a large fraction of the contamination hazard in the first weeks in the Fukushima nuclear crisis. This is because I-131 is a major uranium, plutonium fission product, comprising nearly 3% of the total products of fission (by weight). See fission product yield for a comparison with other radioactive fission products. I-131 is also a major fission product of uranium-233, produced from thorium.