Technetium (₄₃Tc) is one of the two elements with Z < 83 that have no stable isotopes; the other such element is promethium. It is primarily artificial, with only trace quantities existing in nature produced by spontaneous fission (there are an estimated 2.5×10⁻¹³ grams of ⁹⁹Tc per gram of pitchblende) or neutron capture by molybdenum. The first isotopes to be synthesized were ⁹⁷Tc and ⁹⁹Tc in 1936, the first artificial element to be produced. The most stable radioisotopes are ⁹⁷Tc (half-life of 4.21 million years), ⁹⁸Tc (half-life: 4.2 million years), and ⁹⁹Tc (half-life: 211,100 years).

Thirty-three other radioisotopes have been characterized with atomic masses ranging from ⁸⁵Tc to ¹²⁰Tc. Most of these have half-lives that are less than an hour; the exceptions are ⁹³Tc (half-life: 2.75 hours), ⁹⁴Tc (half-life: 4.883 hours), ⁹⁵Tc (half-life: 20 hours), and ⁹⁶Tc (half-life: 4.28 days).

Technetium also has numerous meta states. ^97mTc is the most stable, with a half-life of 91.0 days (0.097 MeV). This is followed by ^95mTc (half-life: 61 days, 0.038 MeV) and ^99mTc (half-life: 6.04 hours, 0.143 MeV). ^99mTc only emits gamma rays, subsequently decaying to ⁹⁹Tc.

For isotopes lighter than ⁹⁸Tc, the primary decay mode is electron capture to isotopes of molybdenum. For the heavier isotopes, the primary mode is beta emission to isotopes of ruthenium, with the exception that ¹⁰⁰Tc can decay both by beta emission and electron capture.

Technetium-99m is the hallmark technetium isotope employed in the nuclear medicine industry. Its low-energy isomeric transition, which yields a gamma-ray at ~140.5 keV, is ideal for imaging using Single Photon Emission Computed Tomography (SPECT). Several technetium isotopes, such as ^94mTc, ^95gTc, and ^96gTc, which are produced via (p,n) reactions using a cyclotron on molybdenum targets, have also been identified as potential Positron Emission Tomography (PET) agents. Technetium-101 has been produced using a D-D fusion-based neutron generator from the ¹⁰⁰Mo(n,γ)¹⁰¹Mo reaction on natural molybdenum and subsequent beta-minus decay of ¹⁰¹Mo to ¹⁰¹Tc. Despite its shorter half-life (i.e., 14.22 min), ¹⁰¹Tc exhibits unique decay characteristics suitable for radioisotope diagnostic or therapeutic procedures, where it has been proposed that its implementation, as a supplement for dual-isotopic imaging or replacement for ^99mTc, could be performed by on-site production and dispensing at the point of patient care.

Technetium-99 is the most common and most readily available isotope, as it is a major fission product from fission of actinides like uranium and plutonium with a fission product yield of 6% or more, and in fact the most significant long-lived fission product. Lighter isotopes of technetium are almost never produced in fission because the initial fission products normally have a higher neutron/proton ratio than is stable for their mass range, and therefore undergo beta decay until reaching the ultimate product. Beta decay of fission products of mass 95–98 stops at the stable isotopes of molybdenum of those masses and does not reach technetium. For mass 100 and greater, the technetium isotopes of those masses are very short-lived and quickly beta decay to isotopes of ruthenium. Therefore, the technetium in spent nuclear fuel is practically all ⁹⁹Tc. In the presence of fast neutrons a small amount of ⁹⁸
Tc will be produced by (n,2n) "knockout" reactions. If nuclear transmutation of fission-derived Technetium or Technetium waste from medical applications is desired, fast neutrons are therefore not desirable as the long lived ⁹⁸
Tc increases rather than reducing the longevity of the radioactivity in the material.

One gram of ⁹⁹Tc produces 6.2×10⁸ disintegrations a second (that is, 0.62 GBq/g).

Technetium has no stable or nearly stable isotopes, and thus a standard atomic weight cannot be given.