稳定同位素（Stable isotope），是指化学元素中，不发生放射性衰变或不易发生放射性衰变的同位素，稳定同位素即使会发生衰变，因半衰期太长而无法测量出。

所有同位素中，只有前66个元素中的146个同位素(最重的为镝164)理论上不会发生放射性衰变（不考虑尚未证实的质子衰变），另外有107个理论上会发生某种放射性衰变（α衰变、β衰变或电子捕获），但尚无法证实其放射性（其中最轻的为氩36）。因此已知的稳定同位素共有253个，分布在80个化学元素中。

在这80种有稳定同位素的元素中，有26个元素只存在一种稳定同位素，因此称为单一同位素元素，其他的元素则有不止一种的稳定同位素，其中锡有10种稳定同位素，是具有最多种稳定同位素的元素。

Stable nuclides are nuclides that are not radioactive and so (unlike radionuclides) do not spontaneously undergo radioactive decay. When such nuclides are referred to in relation to specific elements, they are usually termed stable isotopes.

The 80 elements with one or more stable isotopes comprise a total of 254 nuclides that have not been known to decay using current equipment (see list at the end of this article). Of these elements, 26 have only one stable isotope; they are thus termed monoisotopic. The rest have more than one stable isotope. Tin has ten stable isotopes, the largest number known for an element.