We found low fire activity in northern Arctic Alaska from ~1000 BCE to 500 CE, and a marked increase post-1950 CE, when it exceeded any levels observed in the preceding millennia. Deepening of water tables and peatland drying associated with permafrost thaw have facilitated woody encroachment, leading to enhanced fire activity. This study highlights that moisture–vegetation–fire feedbacks significantly influence tundra fire regimes, with implications for ongoing Arctic greening and warming.