Thwaites Glacier (TG), in West Antarctica, is potentially unstable and may contribute significantly to sea-level rise as global warming continues. Using satellite data, we show that Thwaites Eastern Ice Shelf, the largest remaining floating extension of TG, has started to accelerate as it fragments along a shear zone. Computer modelling does not indicate that fragmentation will lead to imminent glacier collapse, but it is clear that major, rapid, and unpredictable changes are underway.