Atmospheric rivers (ARs) transport concentrated fluxes of heat and moisture poleward, driving temperature and precipitation extremes. Yet, their vertical structure in the High Arctic – where small thermodynamic perturbations govern rain-snow partitioning and cryospheric response – remains poorly constrained. Here we present atmospheric vapour isotope, radiosonde, and meteorological observations from Svalbard during a record-setting AR in March 2022. The AR developed in the northwest Atlantic when a deep "bomb" cyclone established a sustained conduit of poleward heat/moisture. Integrated vapour transport exceeded 450 kg m⁻¹ s⁻¹, with warming and enhanced moisture emerging ~2–6 km aloft before deepening through the lower-troposphere, tripling near-surface humidity. On 15 March, air temperatures rose to 5.6 °C accompanied by 43.9 mm rainfall – the highest daily March total on record. Concurrently, vapour δ¹⁸O (d-excess) attained its campaign maximum (minimum) and marine aerosol (Na⁺) concentrations spiked, constraining the geochemical signature of Atlantic moisture advection. The two-day AR event delivered ~0.5 Gt snowfall across Svalbard, locally equivalent to over 8% of net 2022 glacier accumulation and offsetting surface mass loss by ~7%. Although rainfall comprised less than one-third of the total precipitation, it impacted 60% of the glacierised terrain, driving winter rain-on-snow melt and densification across lower-elevation areas and altering snowpack structure. Our study underscores the vulnerability of Svalbard and other glacierised Arctic archipelagos to intensifying poleward moisture and heat transport by ARs, with substantial but nuanced impacts on glacier surface energy budget and mass balance through the delivery of anomalous winter rainfall, snowfall, and latent heat.