Naturally occurring niobium (₄₁Nb) is composed of one stable isotope (⁹³Nb). The most stable radioisotope is ⁹²Nb with a half-life of 34.7 million years. The next longest-lived niobium isotopes are ⁹⁴Nb (half-life: 20,300 years) and ⁹¹Nb with a half-life of 680 years. There is also a meta state of ⁹³Nb at 31 keV whose half-life is 16.13 years. Twenty-seven other radioisotopes have been characterized. Most of these have half-lives that are less than two hours, except ⁹⁵Nb (35 days), ⁹⁶Nb (23.4 hours) and ⁹⁰Nb (14.6 hours). The primary decay mode before stable ⁹³Nb is electron capture and the primary mode after is beta emission with some neutron emission occurring in ^104–110Nb.

Only ⁹⁵Nb (35 days) and ⁹⁷Nb (72 minutes) and heavier isotopes (half-lives in seconds) are fission products in significant quantity, as the other isotopes are shadowed by stable or very long-lived (⁹³Zr) isotopes of the preceding element zirconium from production via beta decay of neutron-rich fission fragments. ⁹⁵Nb is the decay product of ⁹⁵Zr (64 days), so disappearance of ⁹⁵Nb in used nuclear fuel is slower than would be expected from its own 35-day half-life alone. Small amounts of other isotopes may be produced as direct fission products.