Naturally occurring lutetium (₇₁Lu) is composed of one stable isotope ¹⁷⁵Lu (97.41% natural abundance) and one long-lived radioisotope, ¹⁷⁶Lu with a half-life of 3.78 × 10¹⁰ years (2.59% natural abundance). Thirty-five radioisotopes have been characterized, with the most stable, besides ¹⁷⁶Lu, being ¹⁷⁴Lu with a half-life of 3.31 years, and ¹⁷³Lu with a half-life of 1.37 years. All of the remaining radioactive isotopes have half-lives that are less than 9 days, and the majority of these have half-lives that are less than half an hour. This element also has 18 meta states, with the most stable being ^177mLu (t_1/2 160.4 days), ^174mLu (t_1/2 142 days) and ^178mLu (t_1/2 23.1 minutes).

The isotopes of lutetium range in mass number from 149 to 184. The primary decay mode before the most abundant stable isotope, ¹⁷⁵Lu, is electron capture (with some alpha and positron emission), and the primary mode after is beta emission. The primary decay products before ¹⁷⁵Lu are isotopes of ytterbium and the primary products after are isotopes of hafnium. All isotopes of lutetium are either radioactive or, in the case of ¹⁷⁵Lu, observationally stable, meaning that ¹⁷⁵Lu is predicted to be radioactive but no actual decay has been observed.