Reconstructing the evolution of sea level during past warmer epochs such as the Pliocene, provides unique insight into the response of sea level and ice sheets to prolonged warming1. While estimates of global mean sea level (GMSL) during this time exist, they vary by several tens of metres2–4, hindering the assessment of past and future ice sheet stability. Here we show that during the mid-Piacenzian Warm Period, which was on average 2–3 °C warmer than pre-industrial5, the GMSL was 16.2 m (most likely, 5.6–19.2 m, 68% uncertainty range) higher than today. During the even warmer Pliocene Climatic Optimum (~4 °C warmer than pre-industrial)6, our results show that GMSL was 23.5 m above present (most probably, 9.0–26.7 m, 68% uncertainty range). We present six GMSL data points, ranging from 4.39 to 3.27 million years ago, that are based on phreatic overgrowths on speleothems from the western Mediterranean (Mallorca, Spain). This record is unique owing to its clear relationship to sea level, its reliable U–Pb ages and its long timespan, which allows us to quantify uncertainties on potential uplift. Our data indicate that ice sheets are very sensitive to warming and provide important calibration targets for future ice sheet models7.