The effect of hydrogen on the plasticity of ultrafine-grained austenite was studied on type 304, 316L, and 310S stainless steels processed by high-pressure torsion at moderate temperature. Like austenitic steels with ordinary grain sizes, the hydrogen-induced ductility loss for the ultrafine grains became more pronounced with decreasing stability of the austenitic phase. For all the steels, the uniform elongation was limited by strengthening due to the ultra grain refinement; the ultrafine-grained 310S stable austenitic steel further exhibited a small local elongation due to a lack of martensitic transformation. The ductility loss due to hydrogenation for 316L steel with an intermediate austenite stability was retained to a moderate level. The ultrafine-grained 304 metastable austenitic steel exhibited a serious ductility loss induced by hydrogen showing localised shear deformation. This suggests that the dynamic martensite transformation plays a crucial role in the hydrogen embrittlement of ultrafine-grained metastable austenitic steel.