Together with the Pine Island glacier (PIG), the Thwaites (TG) and Smith (SG) glaciers are the principal drainage systems of the Amundsen Sea (AS) sector of Western Antarctica. Here we use satellite radar altimetry and interferometry to show that a rapid thinning of ice has occurred within the fastest flowing sections of all AS outlet glaciers. The pattern of thinning extends to distances greater than 150 km inland. Between 1991 and 2001, the TG and SG thinned by more than 25 and 45 m at their grounding lines, and a total of 154 ± 16 km³ of ice (or 0.43 mm of eustatic sea level rise) was lost from the AS sector glaciers to the ocean. We show that the thickness changes may have caused the PIG, TG, and SG to retreat inland by over 8, 4, and 7 km respectively, in line with independent estimates of grounding line migration.