We compare multi-ice core data with δ18O model output for the early last interglacial Antarctic sea ice minimum. The spatial pattern of δ18O across Antarctica is sensitive to the spatial pattern of sea ice retreat. Local sea ice retreat increases the proportion of winter precipitation, depleting δ18O at ice core sites. However, retreat also enriches δ18O because of the reduced source-to-site distance for atmospheric vapor. The joint overall effect is for δ18O to increase as sea ice is reduced. Our data-model comparison indicates a winter sea ice retreat of 67, 59, and 43% relative to preindustrial in the Atlantic, Indian, and Pacific sectors of the Southern Ocean. A compilation of Southern Ocean sea ice proxy data provides weak support for this reconstruction. However, most published marine core sites are located too far north of the 128,000 years B.P. sea ice edge, preventing independent corroboration for this sea ice reconstruction. Plain Language Summary The Antarctic isotope and temperature maximum, which occurred approximately 128,000 years before present (B.P.) during the warmer than present last interglacial period, is associated with a major retreat of Antarctic sea ice. Understanding the details of this major sea ice retreat is crucial in order to understand the sensitivity of the Southern Hemisphere sea ice system and to evaluate the performance of climate model simulations in response to future warming. This work uses a multi-ice and ocean core data-model evaluation to assess the magnitude and spatial pattern of this sea ice retreat. Our results suggest that sea ice retreat was greatest in the Atlantic and Indian sectors of the Southern Ocean and less in the Pacific sector. These results may have had serious implications for the stability of marine terminating glaciers around the Antarctic Ice Sheet and their contribution to the last interglacial sea level rise. These results also support a hypothesized slowdown in northward ocean heat transport during the early last interglacial.