Blue petrels (Halobaena caerulea Gmelin) rapidly moult their flight feathers in Antarctic waters in February–April, immediately following the breeding season, yet the behaviour of moulting birds at sea has not been described. We observed large numbers of moulting blue petrels off West Antarctica from 67–71°S and 78–119°W in mid-February 2017. Most of these birds probably breed at the Diego Ramirez archipelago, southwest of Cape Horn, which is the closest colony to this area. Moulting petrels often sit on the water in dense flocks, just outside the marginal ice zone, at sea temperatures of -0.7 to 0.9°C. Wing moult is intense, with 7–8 inner primaries (62–75% of primary length and 55–69% of primary mass), their corresponding primary coverts and all greater secondary coverts being grown at the same time. Moulting petrels need a reliable food source during this energetically demanding period, so the waters off West Antarctica are probably crucial for the Diego Ramirez population, which makes up more than half of the world's blue petrels.