Naturally occurring iodine (₅₃I) consists of one stable isotope, ¹²⁷I, and is a mononuclidic element for atomic weight. Radioisotopes of iodine are known from ¹⁰⁸I to ¹⁴⁷I.

The longest-lived of those, ¹²⁹I, has a half-life of 16.14 million years, which is too short for it to exist as a primordial nuclide. It is, however, found in nature as a trace isotope and universally