ABSTRACT Radioactive strontium ( ⁹⁰ Sr) leaked into saline environments, including the ocean, from the Fukushima Daiichi Nuclear Power Plant after a nuclear accident. Since the removal of ⁹⁰ Sr using general adsorbents (e.g., zeolite) is not efficient at high salinity, a suitable alternative immobilization method is necessary. Therefore, we incorporated soluble Sr into biogenic carbonate minerals generated by urease-producing microorganisms from a saline solution. An isolate, Bacillus sp. strain TK2d, from marine sediment removed >99% of Sr after contact for 4 days in a saline solution (1.0 × 10 ⁻³ mol liter ⁻¹ of Sr, 10% marine broth, and 3% [wt/vol] NaCl). Transmission electron microscopy and energy-dispersive X-ray spectroscopy showed that Sr and Ca accumulated as phosphate minerals inside the cells and adsorbed at the cell surface at 2 days of cultivation, and then carbonate minerals containing Sr and Ca developed outside the cells after 2 days. Energy-dispersive spectroscopy revealed that Sr, but not Mg, was present in the carbonate minerals even after 8 days. X-ray absorption fine-structure analyses showed that a portion of the soluble Sr changed its chemical state to strontianite (SrCO ₃ ) in biogenic carbonate minerals. These results indicated that soluble Sr was selectively solidified into biogenic carbonate minerals by the TK2d strain in highly saline environments. IMPORTANCE Radioactive nuclides ( ¹³⁴ Cs, ¹³⁷ Cs, and ⁹⁰ Sr) leaked into saline environments, including the ocean, from the Fukushima Daiichi Nuclear Power Plant accident. Since the removal of ⁹⁰ Sr using general adsorbents, such as zeolite, is not efficient at high salinity, a suitable alternative immobilization method is necessary. Utilizing the known concept that radioactive ⁹⁰ Sr is incorporated into bones by biomineralization, we got the idea of removing ⁹⁰ Sr via incorporation into biominerals. In this study, we revealed the ability of the isolated ureolytic bacterium to remove Sr under high-salinity conditions and the mechanism of Sr incorporation into biogenic calcium carbonate over a longer duration. These findings indicated the mechanism of the biomineralization by the urease-producing bacterium and the possibility of the biomineralization application for a new purification method for ⁹⁰ Sr in highly saline environments.