There are 39 known isotopes and 17 nuclear isomers of tellurium (₅₂Te), with atomic masses that range from 104 to 142. These are listed in the table below.

Naturally-occurring tellurium on Earth consists of eight isotopes. Two of these have been found to be radioactive: ¹²⁸Te and ¹³⁰Te undergo double beta decay with half-lives of, respectively, 2.2×10²⁴ (2.2 septillion) years (the longest half-life of all nuclides proven to be radioactive) and 8.2×10²⁰ (820 quintillion) years. The longest-lived artificial radioisotope of tellurium is ¹²¹Te with a half-life of about 19 days. Several nuclear isomers have longer half-lives, the longest being ^121mTe with a half-life of 154 days.

The very-long-lived radioisotopes ¹²⁸Te and ¹³⁰Te are the two most common isotopes of tellurium. Of elements with at least one stable isotope, only indium and rhenium likewise have a radioisotope in greater abundance than a stable one.

It has been claimed that electron capture of ¹²³Te was observed, but the recent measurements of the same team have disproved this. The half-life of ¹²³Te is longer than 9.2 × 10¹⁶ years, and probably much longer.

¹²⁴Te can be used as a starting material in the production of radionuclides by a cyclotron or other particle accelerators. Some common radionuclides that can be produced from tellurium-124 are iodine-123 and iodine-124.

The short-lived isotope ¹³⁵Te (half-life 19 seconds) is produced as a fission product in nuclear reactors. It decays, via two beta decays, to ¹³⁵Xe, the most powerful known neutron absorber, and the cause of the iodine pit phenomenon.

With the exception of beryllium, tellurium is the lightest element observed to commonly undergo alpha decay, with isotopes ¹⁰⁴Te to ¹⁰⁹Te being seen to undergo this mode of decay. Some lighter elements, namely those in the vicinity of ⁸Be, have isotopes with delayed alpha emission (following proton or beta emission) as a rare branch.