There are yet unidentified sources of fossil methane (CH4) in the atmosphere. Mud volcanoes (MVs) are a potentially significant but poorly quantified geologic source of fossil hydrocarbon gases and CO2 to the atmosphere and the ocean not included in the current models of sources and sinks. Our statistical analysis of 36 previous measurements and estimates of gas flux from individual MVs suggests that the global gas flux may be as high as ~33 Tg yr-1 (~15.9 Tg yr-1 during quiescent periods plus ~17.1 Tg yr-1 during eruptions). Onshore and shallow offshore MVs are estimated to contribute ~6 Tg yr-1 of greenhouse gases directly to the atmosphere. MVs may contribute ~9% of fossil CH4 missing in the modern atmospheric CH4 budget, and ~12% in the preindustrial budget. Large volumes (~27 Tg yr-1) of gas may escape from deep-water MVs, suggesting that global gas flux from the seafloor may be underestimated.