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Managing nitrogen and phosphorus pollution in high-salinity wastewater is a critical challenge for sustainable aquaculture and environmental protection. In this study, a novel salt-tolerant strain, Pseudomonas sediminis D4, was isolated from a brackish water environment. This strain exhibited simultaneous heterotrophic nitrification–aerobic denitrification and phosphorus removal capabilities. Biosafety assays demonstrated that the strain was antibiotic-sensitive and safe for aquatic environments. The optimal conditions for nitrogen and phosphate removal of strain D4 were carbon/nitrogen (C/N) ratio 10, phosphorus/nitrogen (P/N) ratio 0.2, pH 7, and temperature 30 °C while using sodium succinate as the carbon source. Under these conditions, strain D4 achieved removal efficiencies of 97.36% for ammonia (NH₄⁺-N), 100.00% for nitrate (NO₃⁻-N), and 98.02% for nitrite (NO₂⁻-N), along with 94.69%, 89.56%, and 97.40% removal of PO₄³⁻P, respectively. The strain exhibited strong salinity tolerance, functioning effectively within a range of 0% to 5% (w/v), and maintaining high nitrogen and phosphorus removal efficiency at a salinity of 3%. Enzyme activity assays verified the existence of key enzymes, such as ammonia nitrogen oxidase, nitrate oxidoreductase, nitrate reductase, nitrite reductase, polyphosphate kinase, and exopolyphosphatase, which are essential for the heterotrophic nitrification-aerobic denitrification and phosphorus removal capabilities of D4. These findings highlight the potential of Pseudomonas sediminis D4 for the biological treatment of high-salinity wastewater. Full article