Boreal ecosystems store twice as much carbon as the atmosphere and warm faster than the global average. The current paradigm based on boreal forests and tundra considers that warming will accelerate boreal carbon loss. However, the warming response of Sphagnum peatlands, storing ~40% of boreal carbon stocks, remains under-investigated. Here by coupling meta-analysis of 735 paired observations from 93 boreal warming studies with mechanistic investigations into two long-term warming experiments in Finnish peatlands, we demonstrate that warming enhances soil carbon accumulation in boreal Sphagnum peatlands. This result sharply contrasts with warming-induced carbon loss from boreal forests and tundra, owing to the unique metabolic response of Sphagnum, leading to increased plant productivity, reduced microbial decomposition and enhanced iron-mediated protection of soil organic matter. Our estimates suggest that warming-induced increase of soil carbon in boreal Sphagnum peatlands (assuming no hydrological changes or plant species shifts) may offset nearly half the boreal forest carbon-sink decline or heterotrophic respiration increases in Arctic tundra under warming. These findings highlight the vital but overlooked role of Sphagnum peatlands in counteracting boreal carbon loss under future warming.