<p>Widespread permafrost thaw—indicated by the thickening of the soil active layer, which undergoes seasonal freeze–thaw cycles above the permafrost—has important implications for the thermal, hydrological and ecological processes of Arctic–boreal ecosystems. Yet, how thaw-driven biophysical and ecological feedbacks alter Arctic–boreal fire regimes remains poorly understood. Here we quantify these feedbacks by integrating long-term observations of soil active layer thickness (1997–2018) with a causal inference model and a space-for-time approach. We show that the thickening of the soil active layer can amplify summer fire regimes across the Arctic–boreal region, resulting in intensified fire weather conditions, increased burned area and higher fire emissions. The fire amplifications were driven by thaw-induced reductions in surface albedo and soil moisture that intensify summer warming and atmospheric aridity, further reinforced by enhanced vegetation growth and the desiccation of soil organic matter. These cascading feedbacks may create a positive fire–climate feedback loop, whereby more burning accelerates carbon release from vegetation and soil, further destabilizing northern ecosystems. Our findings highlight the potential escalating fire and climate risks initiated by permafrost thaw, emphasizing the urgent need for immediate climatic action to mitigate climate change and the compounding impacts of wildfires and permafrost degradation.</p>