Indium (₄₉In) consists of two primordial nuclides, with the most common (~ 95.7%) nuclide (¹¹⁵In) being measurably though weakly radioactive. Its spin-forbidden decay has a half-life of 4.41×10¹⁴ years.

The stable isotope ¹¹³In is only 4.3% of naturally occurring indium. Among elements with a known stable isotope, only tellurium and rhenium similarly occur with a stable isotope in lower abundance than the long-lived radioactive isotope. Other than ¹¹⁵In, the longest-lived radioisotope is ¹¹¹In, with a half-life of 2.8047 days. All other radioisotopes have half-lives less than a day. This element also has 47 isomers, the longest-lived being ^114m1In, with a half-life of 49.51 days. All other meta-states have half-lives less than a day, most less than an hour, and many measured in milliseconds or less.

Indium-111 is used medically in nuclear imaging, as a radiotracer nuclide tag for gamma camera localization of protein radiopharmaceuticals, such as In-111-labeled octreotide, which binds to receptors on certain endocrine tumors (Octreoscan). Indium-111 is also used in indium white blood cell scans, which use nuclear medical techniques to search for hidden infections.