The common whelk, Buccinum undatum, is a commercially important gastropod found throughout the North Atlantic. One method of age and life history analysis for gastropod species is the use of oxygen isotope ratio (δ18O) measurements from their shells, which is a well-established technique for the reconstruction of historical seawater temperatures at the time of shell biomineralization. Palaeotemperature calibrations have been developed for different types of calcium carbonate as well as species-specific equations to produce the most accurate seawater temperature reconstructions. Here we investigate the four-layer internal structure of B. undatum shells and confirm an aragonite composition using Micro-Raman Spectroscopy (MRS). We then calibrate a species-specific palaeotemperature equation for this gastropod species. This was achieved through the isotopic analysis of shells from laboratory reared specimens of known provenance reared at specific seawater temperatures to produce the following: t(°C) = 14.96 (± 0.15) – 4.94 (± 0.22) x (δ18Oshell – δ18Owater) The calibrated equation differs significantly from previously published data derived from both aragonite and calcite. An offset of 1.04‰ (± 0.41‰) was discovered between observed δ18OShell values and those expected under equilibrium, suggesting a species-specific vital effect. The calibrated equation was used to reconstruct accurate, high resolution historical seawater temperatures from three sites across the U.K. (Shetland, the Menai Strait and Jersey). With this new accurate calibration, both modern and fossil B. undatum shells now have the potential to be employed as high-resolution archives of recent and historical seawater temperature.