Permafrost thaw in the Arctic enables the biogeochemical transformation of vast stores of organic carbon into carbon dioxide (CO2). This CO2 release has significant implications for climate feedbacks, yet the potential counterbalance from CO2 fixation via chemical weathering of minerals exposed by thawing permafrost is entirely unstudied. We show that thermokarst in the western Canadian Arctic can enable rapid weathering of carbonate tills, driven by sulfuric acid from sulfide oxidation. Unlike carbonic acid‐driven weathering, this caused significant and previously undocumented CO2 production and outgassing in headwater streams. Increasing riverine solute fluxes correspond with long‐term intensification of thermokarst and reflect the regional predominance of sulfuric acid‐driven carbonate weathering. We conclude that thermokarst‐enhanced mineral weathering has potential to profoundly disrupt Arctic freshwater carbon cycling. While thermokarst and sulfuric acid‐driven carbonate weathering in the western Canadian Arctic amplify CO2 release, regional variation in sulfide oxidation will moderate the effects on the permafrost carbon‐climate feedback. Plain Language Summary In the Arctic, perennially frozen ground (permafrost) in previously glaciated regions stores abundant minerals and is often ice‐rich. Therefore, this permafrost can rapidly thaw and collapse, resulting in thermokarst and exposing minerals to breakdown by chemical weathering. Mineral weathering by carbonic acid fixes CO2, making it less likely to enter the atmosphere. However, the effect of thermokarst on mineral weathering, carbon cycling, and rising atmospheric CO2 levels is unknown. We show thermokarst enhances weathering in streams in the western Canadian Arctic can rapidly produce significant and previously undocumented CO2 because carbonate weathering in this region is driven by sulfuric acid (from weathering of sulfide minerals) instead of carbonic acid. Long‐term river chemistry reveals that this weathering is intensifying as thermokarst accelerates. Across the Arctic, increasing thermokarst will profoundly impact freshwater carbon cycling, yet the influence of weathering on climate feedbacks will depend on regional variation in the mineral composition of permafrost soils. Key Points Permafrost thaw‐driven ground collapse (thermokarst) in the western Canadian Arctic enhances carbonate weathering locally and regionally This weathering is driven by sulfuric acid and rapidly produces significant, previously undocumented CO2 Carbonic acid‐driven weathering elsewhere can consume CO2, yet thermokarst effects on weathering, carbon cycle, and climate are unstudied