Hydrogen, produced by water radiolysis, has been suggested to support microbial communities on Mars. We quantitatively assess the potential magnitude of radiolytic H2 production in wet martian environments (the ancient surface and the present subsurface) based on the radionuclide compositions of (1) eight proposed Mars 2020 landing sites, and (2) three sites that individually yield the highest or lowest calculated radiolytic H2 production rates on Mars. For the proposed landing sites, calculated H2 production rates vary by a factor of ∼1.6, while the three comparison sites differ by a factor of ∼6. Rates in wet martian sediment and microfractured rock are comparable with rates in terrestrial environments that harbor low concentrations of microbial life (e.g., subseafloor basalt). Calculated H2 production rates for low-porosity (<35%), fine-grained martian sediment (0.12–1.2 nM/year) are mostly higher than rates for South Pacific subseafloor basalt (∼0.02–0.6 nM/year). Production rates in martian high-porosity sediment (>35%) and microfractured (1 μm) hard rock (0.03 to <0.71 nM/year) are generally similar to rates in South Pacific basalt, while yields for larger martian fractures (1 and 10 cm) are one to two orders of magnitude lower (<0.01 nM/year). If minerals or brine that amplify radiolytic H2 production rates are present, H2 yields exceed the calculated rates.