The mid-Pliocene warm period (MPWP, 3.3–2.9 Ma), with reconstructed atmospheric pCO2 of 350–450 ppm, represents a potential analogue for climate change in the near future. Current highly cited estimates place MPWP maximum global mean sea level (GMSL) at 21 ± 10 m above modern, requiring total loss of the Greenland and marine West Antarctic Ice Sheets and a substantial loss of the East Antarctic Ice Sheet, with only a concurrent 2–3 °C rise in global temperature. Many estimates of Pliocene GMSL are based on the partitioning of oxygen isotope records from benthic foraminifera (δ18Ob) into changes in deep-sea temperatures and terrestrial ice sheets. These isotopic budgets are underpinned by the assumption that the δ18O of Antarctic ice (δ18Oi) was the same in the Pliocene as it is today, and while the sensitivity of δ18Ob to changing meltwater δ18O has been previously considered, these analyses neglect conservation of 18O/16O in the ocean-ice system. Using well-calibrated δ18O-temperature relationships for Antarctic precipitation along with estimates of Pliocene Antarctic surface temperatures, we argue that the δ18Oi of the Pliocene Antarctic ice sheet was at minimum 1‰–4‰ higher than present. Assuming conservation of 18O/16O in the ocean-ice system, this requires lower Pliocene seawater δ18O without a corresponding change in ice sheet mass. This effect alone accounts for 5%–20% of the δ18Ob difference between the MPWP interglacials and the modern. With this amended isotope budget, we present a new Pliocene GMSL estimate of 9–13.5 m above modern, which suggests that the East Antarctic Ice Sheet is less sensitive to radiative forcing than previously inferred from the geologic record.