We present new data about the chemical and structural characteristics of bauxite residue (BR) from Greek Al industry, using a combination of microscopic, analytical, and spectroscopic techniques. SEM-EDS indicated a homogeneous dominant “Al-Fe-Ca-Ti-Si-Na-Cr matrix”, appearing at the microscale. The bulk chemical analyses showed considerable levels of Th (111 μg g−1), along with minor U (15 μg g−1), which are responsible for radioactivity (355 and 133 Bq kg−1 for 232Th and 238U, respectively) with a total dose rate of 295 nGy h−1. Leaching experiments, in conjunction with SF-ICP-MS, using Mediterranean seawater from Greece, indicated significant release of V, depending on S/L ratio, and negligible release of Th at least after 12 months leaching. STEM-EDS/EELS & HR-STEM-HAADF study of the leached BR at the nanoscale revealed that the significant immobility of Th4+ is due to its incorporation into an insoluble perovskite-type phase with major composition of Ca0.8Na0.2TiO3 and crystallites observed in nanoscale. The Th LIII-edge EXAFS spectra demonstrated that Th4+ ions, which are hosted in this novel nano-perovskite of BR, occupy ... mehrCa2+ sites, rather than Ti4+ sites. That is most likely the reason of no Th release in Mediterranean seawater.