In situ electrochemical nanoindentation (ECNI) was used to study the effect of hydrogen on the mechanical properties of Fe-22Mn-0.6C TWIP steel at different potentials. The (110) and (100) oriented grains were tested in this study. The pop-in load, pop-in width, and nano-hardness data were extracted and analyzed. It is clearly shown that the pop-in load and width were reduced while the hardness was increased by charged hydrogen for both grains. Higher cathodic potential caused higher hydrogen concentration, leading to a more obvious change of nanomechanical properties. It is proposed that the nucleation energy for homogeneous dislocation nucleation (HDN) was reduced by the diffusive hydrogen. In addition, the results also show that (110) grain has higher hydrogen sensitivity than (100) grain.