Plant growth promoting bacteria (PGPB) contribute significantly to sustainable agriculture to enhance nutrient availability, regulating plant hormones, and improving tolerance to environmental stresses. This review provides a detailed examination of the mechanistic basis of PGPB-mediated growth promotion, including nitrogen fixation by symbiotic and free-living diazotrophs, solubilization of phosphorus and zinc through microbial metabolism, iron acquisition via siderophore production. The capacity of PGPB to modulate phytohormone levels, particularly auxins, cytokinins, gibberellins, and ethylene, plays a central role in optimizing plant growth and stress adaptation. Despite their multifunctionality, the practical application of PGPB is constrained with inconsistent field performance caused by ecological and environmental factors. Advances in microbial formulation, co-inoculation strategies, and selection of naturally efficient strains are discussed as potential solutions. This is to emphasize the necessity of integrating microbial ecology with agronomic practices with a view to fully realizing the benefits of PGPB in sustainable crop production.