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Helheim Glacier, one of the largest marine-terminating outlet glaciers draining the Greenland Ice Sheet, underwent significant retreat and acceleration in the early-2000s, accounting for an appreciable proportion of the ice sheet’s mass loss during that period. Using a range of remotely sensed datasets, we show that despite a subsequent re-advance, the glacier has continued to lose mass and thin, and has retreated inland of the retracted position occupied in 2005. Critically, the near-terminus is up to 100 m thinner than during 2005, and the front 5 km is within 25-50 m of flotation, with retrograde bed slopes extending several kilometres inland of the terminus. The neighbouring Fenris and Midgard Glaciers have both undergone recent large-scale and rapid retreat once their near-terminus regions began to float, suggesting that under projected climate warming and associated glacier thinning, Helheim Glacier is poised to pass a threshold whereby the near-terminus region will retreat rapidly.