Knowledge about the capacity and mechanisms of thorium uptake in anhydrite is required for not only the safe management of radioactive rare earth element (REE) mine tailings but potential recovery/remediation of this radionuclide from these waste materials for the fourth-generation nuclear power plants. In this study, we investigated Th-doped anhydrite synthesized in both aqueous and sulfuric acid solutions, including samples grown in conditions simulating the Sulfuric Acid Roasting Method (SARM) with monazite-(Ce) ores, at a wide range of temperatures and pH values. Inductively coupled plasma mass spectrometry analyses show samples grown hydrothermally at 353–473 K and pH = 0.3–3 contain 2310–8880 mg/L Th, and 1780 mg/L Th when anhydrite was synthesized by evaporation at 551 K and pH = 0.3. Synchrotron Th LIII-edge X-ray absorption spectroscopy reveals that lattice-bound Th in anhydrite, via the substitution Th4+ + O2− ⇄ Ca2+ with an interstitial O2− ion, can account for at least 1780 mg/L Th, while Th-rich phases such as thorianite and thorite are present in those with still higher Th contents. Therefore, lattice-bound Th in anhydrite must be considered for safe management strategies and future extraction technologies for the recovery/remediation of Th from radioactive REE mine tailings. SynopsisThis study suggests that anhydrite is a major carrier of thorium in the tailings of rare earth element mines. Recovery of thorium from tailings not only reduces their radioactivity, but represents a valuable resource for nuclear fuel production.