The uptake of anthropogenic CO2 by the ocean decreases seawater pH and carbonate mineral aragonite saturation state (Ωarag), a process known as Ocean Acidification (OA). This can be detrimental to marine organisms and ecosystems. The Arctic Ocean is particularly sensitive to climate change and aragonite is expected to become undersaturated (Ωarag < 1) there sooner than in other oceans. However, the extent and expansion rate of OA in this region are still unknown. Here we show that, between the 1990s and 2010, low Ωarag waters have expanded northwards at least 5°, to 85° N, and deepened 100 m, to 250 m depth. Data from trans-western Arctic Ocean cruises show that Ωarag < 1 water has increased in the upper 250 m from 5% to 31% of the total area north of 70° N. Tracer data and model simulations suggest that increased Pacific Winter Water transport, driven by an anomalous circulation pattern and sea-ice retreat, is primarily responsible for the expansion, although local carbon recycling and anthropogenic CO2 uptake have also contributed. These results indicate more rapid acidification is occurring in the Arctic Ocean than the Pacific and Atlantic oceans, with the western Arctic Ocean the first open-ocean region with large-scale expansion of `acidified’ water directly observed in the upper water column.