Abstract: Salinity stress is among of the most severe abiotic stresses threatening global agricultural production, impairing crop yield and quality via multiple mechanisms, including the disruption of soil water balance, induction of ion toxicity, oxidative damage, and competition for nutrient elements. As core members of the plant microbiome, plant growth-promoting rhizobacteria (PGPRs) and plant growth-promoting endophytes (PGPEs) have been extensively studied in recent years with respect to their molecular mechanisms in regulating plant salt stress resistance. In this article, we systematically reviewed the key mechanisms whereby PGPRs and PGPEs enhance plant salt tolerance via multidimensional interactions. Firstly, by synthesizing plant hormones, microbes can modulate plant auxin signaling pathways, gibberellin-mediated cell elongation, and abscisic acid-induced stomatal responses, thereby alleviating the physiological inhibition caused by salinity stress. Secondly, by secreting antioxidant enzymes and non-enzymatic antioxidants, microbes can effectively scavenge reactive oxygen species, mitigate oxidative damage, and maintain cellular metabolic homeostasis. Thirdly, by regulating the expression of ion transport proteins, microbes promote Na⁺ efflux and K⁺ and Ca²⁺ uptake, optimize the K⁺/Na⁺ equilibrium, and reduce ion toxicity. In addition, microbial exopolysaccharides provide synergistic protection by chelating Na⁺ and improving the rhizosphere microenvironment, whereas volatile organic compounds (VOCs) modulate photosynthetic efficiency, osmotic balance, and systemic resistance. Notably, endophytic fungi can activate mitogen-activated protein kinase (MAPK) signaling pathways and the high osmolarity glycerol (HOG-MAPK) pathway, thereby inducing the phosphorylation of transcription factors and upregulating stress-responsive genes to facilitate epigenetic regulation and metabolic reprogramming. Although previous studies have demonstrated that microbes can enhance plant adaptability via the regulation of root architecture, nitrogen/phosphorus metabolism, and signaling molecules, the molecular mechanisms of microbe-derived signal peptides (SPMs) and VOCs in salt stress responses have not yet to be sufficiently determined, particularly how they regulate plant hormone synthesis or interact with pattern recognition receptor signaling pathways via long-distance signaling. Future research should integrate multi-omics technologies to decipher the complex networks of plant-microbe interactions, develop salt tolerance enhancement strategies using synthetic microbial communities, and assess the potential utility of SPMs and VOCs as biosensors or precision agricultural tools. In this review, we aim to provide theoretical foundations and practical directions for microbiome resource utilization in the farming of saline-alkali soils, salt-resistant crop variety improvement, and intelligent environmental monitoring technologies.