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We found boron doping can substantially reduce the ductility-loss in the CrCoNi medium-entropy alloy after gas-hydrogen charging, from ∼71% to ∼46%, while the fracture mode transfers from predominantly intergranular to ductile transgranular dominated. The two alloys have no difference in phase-structures (single face-center-cubic), grain sizes, and grain-boundary (GB) characters. However, atom probe tomography identified apparent GB decoration of boron up to 1.5 at.% and nanometer-scaled scattered borides in boron-doped CrCoNi. Such local chemical difference leads to enhanced GB cohesion and reduced hydrogen diffusivity along GBs, resulting in improved immunity against hydrogen-embrittlement and suppressed mechanical degradation in the boron-doped CrCoNi alloy.