Molecular hydrogen (H2) is an abundant and reactive constituent of Earth's atmosphere, with links to climate and air quality. Anthropogenic emissions of H2 are expected to rise as the use of H2 as an energy source increases. Documenting past variations in atmospheric H2 will help to validate current understanding of the global H2 budget. The modern instrumental record begins in the 1980s; there is little information about atmospheric H2 prior to that time. Here, we use firn air measurements from a 2001 South Pole campaign to reconstruct atmospheric H2 levels over the 20th century. Inversion of the measurements indicates that H2 over South Pole has increased from 350–540 ppb from 1910–2000. A biogeochemical box model indicates that the atmospheric burden of H2 increased by 37% over that time. The rise in H2 is consistent with increasing H2 emissions from fossil fuel combustion and increasing atmospheric production from methane oxidation.