The present study aimed to identify arsenic (As)-resistant bacterial strains that can be used to mitigate arsenic stress. A bacterium Bacillus mycoides NR5 having As tolerance limit of 1100 mg L -1 was isolated from Nag River, Maharashtra, India. It was also equipped with plant growth-promoting (PGP) attributes like P solubilization, siderophores, ammonia and nitrate reduction, with added antibiotic tolerance. Further, Scanning Electron Microscopy (SEM) and Transmission Electron Micrograph (TEM) suggested biosorption as possible mechanisms of arsenic tolerance. A strong peak in FTIR spectra at 3379.0 corresponding to amine in As-treated NR5 also indicated metal interaction with cell surface protein. Amplification of the arsenic reductase gene in NR5 further suggested intracellular transformation of As speciation. Moreover, the As tolerance capability of NR5 was shown in spinach plants in which the bacterium effectively mitigated 25ppm As by the production of defence-related proline molecules. Evidence from SEM, TEM and FTIR, concluded biosorption is possibly the primary mechanism of As tolerance in NR5 along with the transformation of arsenic. B. mycoides NR5 with PGP attributes, high As tolerance and antibiotic resistance mediated enhanced As tolerance in spinach plants advocated that the strain can be a better choice for As bioremediation in contaminated agricultural soil and water.