Soil salinity is a major environmental constraint affecting agricultural production systems. Boron (B) is an essential micronutrient for plants. This work was aimed to reveal the mechanistic basis of amelioration of detrimental effects of salinity on plant performance by boron. Barley (Hordeum vulgare) plants were exposed to different boron (H3BO3, from 0 to 1 mM) and salt (NaCl, 0 and 100 mM) treatments demonstrating salt-tolerant phenotype in the presence of B. This was manifested by the significant increase in leaf and root fresh weight, enhanced CO2 assimilation, intercellular CO2 concentration, and stomatal conductance. A significant reduction in root and shoot Na content combined with higher amounts of K+ in the presence of B provided more optimal tissue K+/Na+ ratios. At the cellular level, B-treated roots showed stronger stimulation of H+-ATPase by NaCl (using net H+ flux as a proxy) and could maintain more negative membrane potential. This enhanced cytosolic potassium retention (by reducing GORK-mediated K+ loss) and provided a driving force for the operation of SOS1 Na+/H+ exchanger explaining whole-plant ionomics data. Boron also affected the sensitivity of K+- and Ca2+- permeable ion channels to reactive oxygen species. Significantly less root K+ loss but more root Ca2+ uptake was observed for +B plants in response to H2O2, indicating the desensitization of K+- permeable channels and the cell type-specific Ca2+- permeable transporters to H2O2, and the role of B in their regulation. Overall, B availability resulted in better salt tolerance in barley, which could be a suitable alternative to crop breeding aimed at enhancing salt tolerance.