Development of thermokarst landforms through the thawing of ice-rich permafrost soils is expected to accelerate in the northern hemisphere due to ongoing climate change. This can damage infrastructure but also drastically impact landscape soil carbon storage and greenhouse gas emissions. Here we present a first circumpolar assessment of the spatial extent and distribution of thermokarst terrain, defined as landscapes where thermokarst landforms either have developed or potentially can develop. We differentiate between wetland, lake and hillslope thermokarst terrain types, and assess regional coverage of each type using geographical information of landscape characteristics, including ground ice content, soil type, topography, biome, and permafrost zone. Each thermokarst terrain type is estimated to occupy 5 to 8% of the northern boreal and tundra permafrost region, but otherwise differ markedly in their spatial distribution and projected exposure to climate change. With high soil organic carbon content, thermokarst terrain is estimated to store a disproportionate 30% of the total permafrost region soil organic carbon stock in the upper 3 meters of soil, and potentially more than half when accounting for deeper carbon stores. This first-order estimate of the distribution of northern thermokarst terrain is an essential step for assessing soil carbon vulnerability to thaw and the magnitude of the permafrost carbon feedback.