The ice sheets of the Amundsen Sea Embayment (ASE) are vulnerable to the marine ice sheet instability (MISI), which would cause irreversible collapse and raise sea levels by over a meter. The uncertain timing and scale of this collapse depend on the complex interaction between ice, ocean, and bedrock dynamics. The mantle beneath parts of the ASE may be much less viscous than the average mantle and able to produce glacial isostatic adjustment (GIA) faster than previously thought. We coupled the BISICLES adaptive mesh refinement ice-sheet model to a viscously responding bedrock to resolve the small spatial and temporal scales of rapid grounding line retreat. Coupling with the weakest possible viscous mantle consistent with regional inversions, we show that Pine Island Glacier's retreat produces a local uplift response that reduces mass lost and slows grounding line retreat by as much as 30% over 150 years. We therefore consider GIA an essential process to model when forecasting centennial ice dynamics in West Antarctica and expect it to significantly alter the dynamics of other marine glaciers, such as Thwaites Glacier nearby.