The effect of hydrogen on thermally activated dislocation movement in high-strength martensitic steel with various amounts of hydrogen was studied using tensile tests and stress relaxation tests to clarify the role of hydrogen in the change of the mechanical properties. Oil-quenched JIS-SCM420 steel with a martensitic microstructure was prepared. After different amounts of hydrogen were introduced by electrochemical charging, tensile tests, and stress relaxation tests were conducted via in-situ hydrogen charging at 30 °C to evaluate the changes in both the nominal stress and the amount of stress relaxation compared with those for non-charged specimens. The resultant shear stress relaxation data were analyzed using nonlinear fitting curves to compare the apparent thermal activation volume of dislocation slipping with/without hydrogen charging. The change in nominal stress increased with increasing amount of hydrogen and was a maximum near the 0.2% proof stress regime. Meanwhile, the absolute value of stress relaxation decreased and the apparent thermal activation volume increased; these values were almost unchanged when the amount of hydrogen was more than 0.27 ppm. These results suggested that hydrogen acts as a barrier to dislocation slipping and causes hardening.