Abstract Siberian wildfires, a major disturbance to ecosystems and carbon cycles, have intensified in recent decades, yet long‐term changes in fire seasonality remain unclear. Here, using a refined fire‐season framework, we identify a pronounced shift in fire season characteristics over eastern Siberia during 2002–2023, marked by substantial lengthening of the fire season and increasingly clustered occurrence of large fires. The fire season extends primarily through delayed termination of 1.48 days/yr into late autumn, associated with the growing persistence of extremely large September fires and more wildfire ignitions in October. We further find that enhanced fire‐weather conditions are the main driver, arising from accumulated drought associated with precipitation deficits in the preceding August–September period. These deficits coincide with persistent high‐pressure anomalies linked to declining Arctic sea ice and subpolar North Atlantic cooling, underscoring the importance of understanding mechanisms and improving early warning of antecedent hydroclimate in shaping high‐latitude fire‐season dynamics.