Hydrogen effects on dislocation nucleation in polycrystalline ferritic Fe-15Cr binary alloy were investigated using in situ electrochemical nanoindentation. Linear elastic finite element calculations taking account of the cubic symmetry of BCC crystals were made to extract the resolved shear stresses on the different slip systems of the indented grains. It is shown that the resolved shear stresses corresponding to the experimentally measured pop-in loads are compatible with homogeneous dislocation nucleation. The resolved shear stress necessary to nucleate dislocations from defect-free crystals was estimated at μ/12-μ/13 without hydrogen. Cathodically charged hydrogen reduces the shear strength of the crystal by 8.5%-19.8%, thus promoting dislocation multiplication.