Naturally occurring rhenium (₇₅Re) is 37.4% ¹⁸⁵Re, which is stable (although it is predicted to decay), and 62.6% ¹⁸⁷Re, which is unstable but has a very long half-life (4.12×10¹⁰ years). Among elements with a known stable isotope, only indium and tellurium similarly occur with a stable isotope in lower abundance than the long-lived radioactive isotope.

There are 33 other unstable isotopes recognized, the longest-lived of which are ¹⁸³Re with a half-life of 70 days, ¹⁸⁴Re with a half-life of 38 days, ¹⁸⁶Re with a half-life of 3.7186 days, ¹⁸²Re with a half-life of 64.0 hours, and ¹⁸⁹Re with a half-life of 24.3 hours. There are also numerous isomers, the longest-lived of which are ^186mRe with a half-life of 200,000 years and ^184mRe with a half-life of 177.25 days. All others have half-lives less than a day.