Although the general patterns of deglacial climate change are relatively well constrained, how, and to what magnitude, large parts of the Southern Hemisphere responded to deglacial forcings remains unknown, particularly for the early part of the last deglaciation. We investigate the timing and magnitude of early deglacial climate change using cosmogenic 10Be surface exposure ages of moraines deposited by glaciers in the Rio Guanaco Valley, adjacent to the Southern Patagonian Ice Field at 50°S. We demonstrate that the beginning of ice retreat from the local last glacial maximum occurred at 19.7 ± 1.1 ka, with significant retreat commencing at 18.9 ± 0.4 ka, concurrent with glacier retreat elsewhere in southern Patagonia and New Zealand and with warming of Southern Hemisphere middle to high latitudes. A third moraine shows that half of the deglacial retreat upvalley had occurred by 17.0 ± 0.3 ka. Equilibrium line altitudes and climate simulations show ∼1.5 °C of warming in southern Patagonia between 18.9 ± 0.4 ka and 17.0 ± 0.3 ka, one-third of the total estimated deglacial warming relative to present. The climate model links this warming to retreat of Northern Hemisphere ice sheets ca. 19 ka through changes in ocean circulation that caused a bipolar seesaw response resulting in Southern Hemisphere warming and driving initial deglaciation across southern Patagonia.