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A new class of materials called high-entropy alloys (HEAs) constitutes multiple principal elements in similar compositional fractions. The equiatomic Fe₂₀Mn₂₀Cr₂₀Co₂₀Ni₂₀ (at%) HEA shows attractive mechanical properties, particularly under cryogenic conditions. Yet, it lacks sufficient yield and ultimate tensile strengths at room temperature. To strengthen these materials, various strategies have been proposed mainly by tuning the composition of the bulk material while no efforts have been made to decorate and strengthen the grain boundaries. Here, we introduce a new HEA design approach that is based on compositionally conditioning the grain boundaries instead of the bulk. We found that as little as 30 ppm of boron doping in single-phase HEAs, more specific in an equiatomic FeMnCrCoNi and in a non-equiatomic Fe₄₀Mn₄₀Cr₁₀Co₁₀ (at%), improves dramatically their mechanical properties, increasing …