Soil salinity is a major abiotic stressor that severely limits global crop productivity. Plant growth-promoting bacteria (PGPB) or plant probiotic bacteria (PPB) offer a sustainable solution for salinity stress by enhancing plant tolerance, primarily through the modulation of the host plant’s antioxidant defense system. This study aimed to isolate and characterize novel halotolerant bacteria from the medicinal plant Senna alexandrina and to elucidate mechanistic role of the most promising isolate in promoting growth and conferring salinity stress tolerance in cucumber (Cucumis sativus). A total of twenty-eight bacterial isolates were obtained (isolated) from S. alexandrina leaves, and four were selected based on high level of phytohormone indole acetic acid (IAA) production and bactericidal activity. Isolate BzAY21 demonstrated the most potent plant-beneficial traits and was molecularly identified via 16S rRNA gene sequencing as Bacillus zhangzhouensis strain YAZ (GenBank accession OR608231). Inoculation of cucumber with B. zhangzhouensis YAZ significantly enhanced overall plant growth and total chlorophyll content compared to non-inoculated controls. Under salinity stress, the isolate BzAY21 markedly improved stress tolerance by boosting key defensive parameters viz. a significant increase in proline accumulation and enhanced activity of antioxidant enzymes, polyphenol oxidase, peroxidase, and catalase. Furthermore, B. zhangzhouensis YAZ exhibited potent bactericidal activity against pathogen Staphylococcus aureus and the plant pathogens Streptomyces scabiei and Erwinia carotovora. Taken together, our results indicate that B. zhangzhouensis YAZ is a highly potential biostimulant/biofertilizer that simultaneously promotes growth and mitigates salinity-induced oxidative stress in cucumber. Future studies should employ multi-omic approaches to validate endophytic establishment and dissect the signaling crosstalk regulating these symbiotic interactions.