Inland waters, including streams and rivers, are active components of the global carbon cycle.Despite the large areal extent of the world’s mountains, the role of mountain streams forglobal carbonfluxes remains elusive. Using recent insights from gas exchange in turbulentstreams, we found that areal CO2evasionfluxes from mountain streams equal or exceedthose reported from tropical and boreal streams, typically regarded as hotspots of aquaticcarbonfluxes. At the regional scale of the Swiss Alps, we present evidence that emitted CO2derives from lithogenic and biogenic sources within the catchment and delivered by thegroundwater to the streams. At a global scale, we estimate the CO2evasion from mountainstreams to 167 ± 1.5 Tg C yr−1, which is high given their relatively low areal contribution to theglobal stream and river networks. Ourfindings shed new light on mountain streams for globalcarbonfluxes.