The Patagonian slab window has been proposed to enhance the solid Earth response to ice mass load changes in the overlying Northern and Southern Patagonian Icefields (NPI and SPI, respectively). Here, we present the first regional seismic velocity model covering the entire north-south extent of the slab window. A slow velocity anomaly in the uppermost mantle indicates warm mantle temperature, low viscosity, and possibly partial melt. Low velocities just below the Moho suggest that the lithospheric mantle has been thermally eroded over the youngest part of the slab window. The slowest part of the anomaly is north of 49°S, implying that the NPI and the northern SPI overlie lower viscosity mantle than the southern SPI. This comprehensive seismic mapping of the slab window provides key evidence supporting the previously hypothesized connection between post-Little Ice Age anthropogenic ice mass loss and rapid geodetically observed glacial isostatic uplift (≥4 cm/ yr). Plain Language Summary A gap in the subducting plate beneath Patagonia has enabled hotter, less viscous mantle material to flow underneath South America. Icefields in the Austral Andes above the gap in the plate have recently been shrinking, removing weight that had caused the continent to flex downward. We use seismic data to image the subsurface structure and find very low seismic velocity within and around the gap, as well as thinning of the rigid South American lithosphere overlying the gap. The low mantle velocity implies that mantle viscosity is also low beneath the shrinking icefields, and low viscosity enables the region to rebound upwards.