Naturally occurring thulium (₆₉Tm) is composed of one stable isotope, ¹⁶⁹Tm (100% natural abundance). Thirty-nine radioisotopes have been characterized, with the most stable being ¹⁷¹Tm with a half-life of 1.92 years, ¹⁷⁰Tm with a half-life of 128.6 days, ¹⁶⁸Tm with a half-life of 93.1 days, and ¹⁶⁷Tm with a half-life of 9.25 days. All of the remaining radioactive isotopes have half-lives that are less than 64 hours, and the majority of these have half-lives that are less than 2 minutes. This element also has 26 meta states, with the most stable being ^164mTm (t_1/2 5.1 minutes), ^160mTm (t_1/2 74.5 seconds) and ^155mTm (t_1/2 45 seconds).

The known isotopes of thulium range from ¹⁴⁴Tm to ¹⁸³Tm. The primary decay mode before the most abundant stable isotope, ¹⁶⁹Tm, is electron capture, and the primary mode after is beta emission. The primary decay products before ¹⁶⁹Tm are erbium isotopes, and the primary products after are ytterbium isotopes. All isotopes of thulium are either radioactive or, in the case of ¹⁶⁹Tm, observationally stable, meaning that ¹⁶⁹Tm is predicted to be radioactive but no actual decay has been observed.

Thulium-170 has a half-life of 128.6 days, decaying by β⁻ decay about 99.87% of the time and electron capture the remaining 0.13% of the time.^[1] Due to its low-energy X-ray emissions, it has been proposed for radiotherapy^[6] and as a source in a radiothermal generator.^[7]