Abstract: This study evaluates the biocontrol efficacy of Bacillus siamensis GXMZU-81 against banana wilt disease and its potential as a plant growth-promoting bacterium. The antagonistic strain GXMZU-81 was isolated using a dual-culture method and exhibited inhibition rates of 70.4% and 72.83% against the mycelial growth and spore germination of Fusarium oxysporum f. sp. cubense tropical race 4 (Foc4), respectively. Morphological, physiological, and biochemical characterization, along with 16S rDNA sequence analysis, identified GXMZU-81 as Bacillus siamensis. The strain demonstrated broad-spectrum antifungal activity and exhibited phosphate-solubilizing and siderophore-producing capabilities. Pot experiments showed that GXMZU-81 achieved 41.06% disease prevention and 26.33% therapeutic efficacy against banana wilt. Additionally, it promoted the growth of banana, maize, and lettuce, with the most pronounced effect observed in lettuce. Optimization of culture conditions indicated that soluble starch and yeast extract were the optimal carbon and nitrogen sources, respectively. These findings expand the microbial resources available for the biocontrol of banana wilt and plant growth promotion, providing a promising candidate for the development of biocontrol agents and biofertilizers.