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Climate change is intensifying the fire regime across Siberia, with the potential to alter carbon combustion and post‐fire carbon re‐accumulation trajectories. Few field‐based estimates of fire severity (e.g., carbon combustion and tree mortality) exist in Siberian larch forests (Larixspp.), which limits our ability to project how an intensified fire regime will affect regional and global climate feedbacks. Here, we present field‐based estimates of fire‐induced tree mortality and carbon loss in eastern Siberian larch forests. Our results suggest that fires in this region result in high tree mortality (means of 83% and 76% at Arctic and subarctic sites, respectively). In both absolute and relative terms, aboveground carbon loss following fire is higher in Siberian larch forests than in North America, but belowground carbon loss is considerably lower. This suggests fundamental differences in wildfire behavior and carbon dynamics between dominant vegetation types across the boreal biome. With climate change, forest fires in Siberia are expected to become more severe and more frequent, which could amplify climate change by transferring carbon from the ecosystem to the atmosphere. Although Siberian larch forests make up 20% of the boreal forest biome by area, scientific understanding of the Siberian larch fire regime is limited because the region is remote and mostly roadless. We collected data from burned and nearby unburned Siberian larch forests to understand the effects of fire on the ecosystem. We found that fires in Siberian larch forests kill, on average, about 75% of trees and result in large carbon losses to the atmosphere. These observations of tree mortality and carbon loss are higher than reported in most satellite‐based studies and demonstrate that fires in Siberian larch forests could contribute to ongoing climate change. Siberian larch forest fires cause high tree mortality despite species traits and stand structure hypothesized to promote low severity firesCompared with boreal North America, fires in Siberian larch forests result in greater aboveground C loss but lower belowground C loss Siberian larch forest fires cause high tree mortality despite species traits and stand structure hypothesized to promote low severity fires Compared with boreal North America, fires in Siberian larch forests result in greater aboveground C loss but lower belowground C loss