Soil salinization is a major constraint on sustainable agriculture worldwide, highlighting the need for stress-tolerant plant growth-promoting rhizobacteria (PGPR) for salt-affected soils. A moderately halophilic and alkali-tolerant bacterium, Staphylococcus succinus NYN-1, was isolated from the rhizosphere soil of the halophyte Suaeda dendroides collected from a highly salinized site in Xinjiang, China. This study aimed to evaluate its salt–alkali tolerance and plant growth-promoting potential through integrated phenotypic characterization, pot experiments, and whole-genome analysis. NYN-1 grew over a broad salinity range [0–15% (w/v)] and pH range (6.0–11.0), and showed plant growth-promoting activities including organic phosphorus mineralization, inorganic phosphate solubilization, potassium solubilization, and NH₄⁺ production. In pot experiments under 300 mM NaCl, inoculation with NYN-1 significantly improved the growth performance of maize (Zea mays L.), cotton (Gossypium hirsutum L.), and sunflower (Helianthus annuus L.). Genome analysis identified multiple Na⁺/H⁺ antiporter-related genes and genes encoding compatible-solute transport systems that are consistent with adaptation to salt–alkali stress. The genome also harbors a broad set of genes related to phosphorus metabolism, as well as other plant growth-promoting functions, including potassium solubilization-related pathways and siderophore biosynthesis. Collectively, these findings identify S. succinus NYN-1 as a promising native halophilic PGPR candidate and a potential microbial resource for developing inoculant strategies in salt-affected agricultural systems.