Thorium (₉₀Th) has seven naturally occurring isotopes but none are stable. One isotope, ²³²Th, is relatively stable, with a half-life of 1.405×10¹⁰ years, considerably longer than the age of the Earth, and even slightly longer than the generally accepted age of the universe. This isotope makes up nearly all natural thorium, so thorium was considered to be mononuclidic. However, in 2013, IUPAC reclassified thorium as binuclidic, due to large amounts of ²³⁰Th in deep seawater. Thorium has a characteristic terrestrial isotopic composition and thus a standard atomic weight can be given.

Thirty-one radioisotopes have been characterized, with the most stable being ²³²Th, ²³⁰Th with a half-life of 75,380 years, ²²⁹Th with a half-life of 7,917 years, and ²²⁸Th with a half-life of 1.92 years. All of the remaining radioactive isotopes have half-lives that are less than thirty days and the majority of these have half-lives that are less than ten minutes. One isotope, ²²⁹Th, has a nuclear isomer (or metastable state) with a remarkably low excitation energy, recently measured to be 8.28 ± 0.17 eV. It has been proposed to perform laser spectroscopy of the ²²⁹Th nucleus and use the low-energy transition for the development of a nuclear clock of extremely high accuracy.

The known isotopes of thorium range in mass number from 207 to 238.