We used Fe-20Mn, Fe-28Mn, Fe-32Mn, and Fe-40Mn alloys. ε-martensite in the Fe-20Mn alloy critically deteriorated the resistance to hydrogen embrittlement owing to α'-martensite. However, when ε-martensite is stable, hydrogen embrittlement susceptibility became low, particularly in the Fe-32Mn alloys, even though the formation of ε-martensite plates assisted boundary cracking. The Fe-40Mn alloys, in which no martensite forms even after fracture, showed higher hydrogen embrittlement susceptibility compared to the Fe-32Mn alloy. Namely, in Fe-Mn binary alloys, the Mn content has an optimal value for hydrogen embrittlement susceptibility because of the following two reasons: 1) The formation of stable ε-martensite seems to have a positive effect in suppressing hydrogen-enhanced localized plasticity, but causes boundary cracking, and 2) an increase in Mn content stabilizes austenite, suppressing martensite-related cracking, but probably decreases the cohesive energy of grain boundaries, causing intergranular cracking. As a consequence, the optimal Mn content was 32 wt.% in the present alloys.