Climate warming is projected to intensify tundra wildfire, with profound implications for permafrost thaw. A major uncertainty is how increased burning will interact with climate change to exacerbate thermokarst (ground-surface collapse resulting from permafrost thaw). Here we show that thermokarst rates increased by ∼60% with warming climate and wildfire from 1950 to 2015 in Arctic Alaska. Wildfire amplified thermokarst over 40+ years, cumulatively creating ∼9 times thermokarst formation as that in unburned tundra. However, thermokarst triggered by repeat burns did not differ from that triggered by single burns, irrespective of time between fires. Our simulation identified climate change as the principal driver of all thermokarst formed during 1950–2015 (4,700 km2) in Arctic Alaska, but wildfire was disproportionately responsible for 10.5% of the thermokarst by burning merely 3.4% of the landscape. These results combined suggest that climate change and wildfire will synergistically accelerate thermokarst as the Arctic transitions in this century.