Naturally occurring ytterbium (₇₀Yb) is composed of seven stable isotopes, ¹⁶⁸Yb–¹⁷⁶Yb, with ¹⁷⁴Yb being the most abundant (31.83% natural abundance). Twenty-seven radioisotopes have been characterized, with the most stable being ¹⁶⁹Yb with a half-life of 32.026 days, ¹⁷⁵Yb with a half-life of 4.185 days, and ¹⁶⁶Yb with a half-life of 56.7 hours. All of the remaining radioactive isotopes have half-lives that are less than 2 hours, and the majority of these have half-lives that are less than 20 minutes. This element also has 12 meta states, with the most stable being ^169mYb (t_1/2 46 seconds).

The isotopes of ytterbium range in atomic weight from 147.967 u (¹⁴⁸Yb) to 180.9562 u (¹⁸¹Yb). The primary decay mode before the most abundant stable isotope, ¹⁷⁴Yb is electron capture, and the primary mode after is beta emission. The primary decay products before ¹⁷⁴Yb are isotopes of thulium, and the primary products after are isotopes of lutetium. Of interest to modern quantum optics, the different ytterbium isotopes follow either Bose–Einstein statistics or Fermi–Dirac statistics, leading to interesting behavior in optical lattices.