Marine Isotope Stage (MIS) 11 was possibly the longest (~423,000-362,000 yr ago) and warmest interglacial of the entire Quaternary Period. This resulted from a special arrangement of Earth's orbital parameters-similar to the present interglacial (MIS 1)-suggesting that MIS 11 provides an analogue for future natural climate forcing and sea levels. Although precise documentation of MIS 11 sea level history is, therefore, crucial - especially considering additional impetus from anthropogenic warming - the maximum MIS 11 sea level remains highly contentious. Estimates from onshore indicators have ranged from below present sea level to about +20. m, resulting from uncertainties in reliability of sea level indicators, estimates of their subsequent displacement by vertical crustal motion and in dating. We used a clear and unambiguous sea level indicator (interface between subtidal and intertidal deposits) in well exposed, regressive raised shoreline deposits along South Africa's southern coast. The coast is stable in terms of tectonic setting and falls within a zone of low predicted glacio-isostatic adjustment (GIA) and low sensitivity for plausible model parameters. The sea level indicators were directly dated using thermally transferred optically stimulated luminescence (TT-OSL), which gave a mean age of ~390. ka, corresponding with MIS 11. We determined a precise maximum elevation of +14. m for the MIS 11 sea level indicator, which corrected for minor crustal uplift and GIA yielded a eustatic sea level of +13. m ±2. m. The complete MIS 11 transgressive/regressive shoreline succession is preserved and the wave-cut platform extends 950. m inland from the present shoreline, indicating a prolonged highstand. We have also documented sea level fluctuations during MIS 11, including an earlier (lower) highstand close to present sea level, which may explain the lower elevation range of some previous estimates. At one locality, MIS 11 deposits form a sea cliff against which MIS 5e sediments (at an elevation of +6.2. m) abut. The +13. m MIS 11 sea level tallies with total melting of the Greenland and West Antarctica ice sheets, regarded as most susceptible to melting. Our data predict notably higher sea levels in the future, regardless of anthropogenic influence on climate. © 2012 Elsevier B.V.