The large marine gastropod Turbo sarmaticus was (and still is) an important food item for coastal foragers along the south coast of South Africa. Opercula of this species are well-preserved in both Holocene and Pleistocene archaeological sites, offering the opportunity to reconstruct past sea surface temperatures from δ¹⁸O of shell carbonate over much of the last glacial cycle. We have analysed δ¹⁸O and δ¹³C in 16 opercula and one shell of modern animals collected alive from an area with good SST records, in order to explore how reliably this species records water temperature through the year and to investigate possible ‘vital effects’ that may influence the isotope record. We find that T. sarmaticus deposits aragonite in both operculum and shell in oxygen isotopic equilibrium with seawater. Operculum δ¹⁸O values of this species reliably reflect water temperatures between c.14 and 24 °C, although they do not always capture temperature extremes, especially in winter, likely due to slower growth under stressful growth conditions. These thresholds probably alter according to factors such as variations in nutrient supply. The averaged maximum and minimum values across the set of sixteen serially sampled opercula very closely approximate the temperatures of the warmest and coolest months and therefore provide a good measure of the seasonal temperature range, and of mean annual SST. δ¹⁸O of edge increments can, to some extent, be used to indicate season of collection, within the broad categories of warmer/cooler seasons.