Carburized gear steel has a high-hardness case layer with excellent wear and fatigue resistance and a low-hardness core with high toughness. Such different microstructures imply different susceptibilities to hydrogen embrittlement (HE). However, a few or no studies have explored the HE behavior of carburized gear steel. Herein, the HE behavior of a vacuum-carburized gear steel 20Cr2Ni4A was investigated via an electrochemical hydrogen-charging and slow strain rate tensile test. For comparison, another group of specimens was prepared by a conventional quenched and tempered (QT) treatment. The volume fraction of retained austenite was significantly higher in the case layer of the carburized specimen (13.8%) than in the core and the QT specimen (4.6%). The retained austenite in the case layer showed a mainly irregular block-type morphology with wide size distribution. The room-temperature diffusible hydrogen content in the hydrogen-charged carburized specimen were almost identical to the QT specimen but the nondiffusible hydrogen content was significantly higher in the former than in the latter. Meanwhile, the hydrogen diffusion coefficient was notably lower in the hydrogen-charged carburized specimen than that in the QT sepcimen because the former retained higher fractions of austenite and cementite. The QT specimen exhibited superior strength and ductility. After hydrogen charging, the strength of the QT specimen remained almost unchanged but the total elongation notably decreased, causing the HE index (HEI), as evidenced using the relative total elongation loss, being 54.3%. Relative to the QT specimen, the carburized specimen achieved a higher tensile strength (increase by 34.6%) but a much lower ductility (total elongation and reduction of area reductions by 66.5% and 92.4%, respectively). The carburized specimen underwent premature brittle fracture before yielding, indicating susceptibility to HE. In fact, the HEI was as high as 90.9%. Mixed intergranular and quasi-cleavage fractures were observed in the surface embrittled region of the hydrogen-charged QT specimen. This region roughly corresponded to the maximum hydrogen diffusion distance. Meanwhile, the hydrogen-charged carburized specimen exhibited an embrittled internal-surface region with a certain width of intergranular fracture, and a long crack had propagated along the circumferential direction near the effective case depth. The microstructure, strength level, and residual stress are thought to mainly explain the abovementioned differences between the carburized and QT specimens.