Conventional wisdom views winter as ecologically dormant, overlooking a critical phase in the annual nitrogen (N) cycle. This review establishes the ‘winter N pump’ as a key conceptual framework, reframing the cold season as an active control point for ecosystem N budgets. Beneath the snowpack, cold-adapted soil microbes drive continuous N transformations, accumulating a substantial hidden N reservoir. Simultaneously, woody plants take up and store N during winter, while herbaceous plants do not resume N acquisition until spring. Spring snowmelt releases this reservoir in a synchronised pulse, and together with the N stored in woody plants, it fuels initial spring growth. Thus, the pump acts as a critical ‘pre-positioning’ mechanism that directly links winter biogeochemical processes to growing-season productivity. Climate change, through winter warming, earlier spring snowmelt, and intensified freeze–thaw cycles, is disrupting this coupling, thereby decoupling the spring N pulse from plant N demand, potentially increasing N losses via leaching and denitrification, particularly in vulnerable ecosystems. Integrating winter N dynamics into ecological models and empirical research is therefore essential to accurately predict future ecosystem productivity, carbon sequestration and biogeochemical feedback. Explicit incorporation of the ‘winter N pump’ concept is crucial for developing strategies to maintain nutrient cycling and ecosystem resilience under global climate change.