Abstract: The speciation of Sr²⁺ influences its environmental behavior and pollution control in marine systems. Coastal seawater salinity exhibits considerable spatiotemporal variation. However, Sr²⁺ speciation patterns across different salinity gradients remain poorly characterized. By combining computational simulations with experimental characterization, this study systematically elucidated the transformation of Sr²⁺ species in simulated seawater across a salinity range of 0.1–30.0. A preliminary investigation into Sr²⁺ speciation in natural seawater was also conducted. Results indicate that in low-salinity systems (0.1), Sr²⁺ primarily binds to medium-molecular-weight (MW) natural organic matter. Increasing salinity progressively inhibits Sr²⁺ binding to both organic matter and particulates. This promotes a shift from bound to free Sr²⁺ species. The dominant binding mechanism transitions from electrostatic interactions to coordination complexation. Coexisting Ca²⁺ and inorganic particles further suppress Sr²⁺-organic matter binding through distinct competitive mechanisms. Applying these findings to natural seawater reveals key characteristics of Bohai Sea samples: high salinity (33), low organic content (TOC = 1.24 mg/L), and elevated Ca²⁺ concentration (349 mg/L). Consequently, free Sr²⁺ ions dominate in this system. The concentrations of Sr²⁺ bound with medium and high-MW organics were both very low, measuring only 9 µg/L each. These findings provide important guidance for controlling radioactive Sr²⁺ contamination in seawater and for the safe utilization of ocean resources.