Beta-decay stable isobars are the set of nuclides which cannot undergo beta decay, that is, the transformation of a neutron to a proton or a proton to a neutron within the nucleus. A subset of these nuclides are also stable with regards to double beta decay or theoretically higher simultaneous beta decay, as they have the lowest energy of all isobars with the same mass number.

This set of nuclides is also known as the line of beta stability, a term already in common use in 1965. This line lies along the bottom of the nuclear valley of stability.

The line of beta stability can be defined mathematically by finding the nuclide with the greatest binding energy for a given mass number, by a model such as the classical semi-empirical mass formula developed by C. F. Weizsäcker. These nuclides are local maxima in terms of binding energy for a given mass number.

All odd mass numbers have only one beta decay stable nuclide.

Among even mass number, five (124, 130, 136, 150, 154) have three beta-stable nuclides. None have more than three; all others have either one or two.

All primordial nuclides are beta decay stable, with the exception of ⁴⁰K, ⁵⁰V, ⁸⁷Rb, ¹¹³Cd, ¹¹⁵In, ¹³⁸La, ¹⁷⁶Lu, and ¹⁸⁷Re. In addition, ¹²³Te and ^180mTa have not been observed to decay, but are believed to undergo beta decay with extremely long half-lives (over 10¹⁵ years). Theoretically, ¹²³Te can only undergo electron capture to ¹²³Sb, whereas ^180mTa can decay in both directions, to ¹⁸⁰Hf or ¹⁸⁰W. Among non-primordial nuclides, there are some other cases of theoretically possible but never-observed beta decay, notably including ²²²Rn and ²⁴⁷Cm (the most stable isotopes of their elements considering all decay modes). Finally, ⁴⁸Ca and ⁹⁶Zr have not been observed to undergo beta decay (theoretically possible for both) which is extremely suppressed, but double beta decay is known for both. Similar suppression of single beta decay occurs also for ¹⁴⁸Gd, a rather short-lived alpha emitter.

All elements up to and including nobelium, except technetium, promethium, and mendelevium, are known to have at least one beta-stable isotope. It is known that technetium and promethium have no beta-stable isotopes; current measurement uncertainties are not enough to say whether mendelevium has them or not.

346 nuclides (including ²⁶⁰Fm whose discovery is unconfirmed) have been definitively identified as beta-stable. Theoretically predicted or experimentally observed double beta decay is shown by arrows, i.e. arrows point toward the lightest-mass isobar. This is sometimes dominated by alpha decay or spontaneous fission, especially for the heavy elements. Observed decay modes are listed as α for alpha decay, SF for spontaneous fission, and n for neutron emission in the special case of ⁵He. For mass 5 there are no bound isobars at all; mass 8 has bound isobars, but the beta-stable ⁸Be is unbound.