New calcite 87Sr/86Sr data for 47 limestones from the metamorphosed and deformed Neoproterozoic–Cambrian Dalradian Supergroup of Scotland and Ireland are used to identify secular trends in seawater 87Sr/86Sr through the Dalradian succession and to constrain its depositional age. Dalradian limestones commonly have Sr >1000 ppm, indicating primary aragonite and marine diagenesis. Low Mn, Mn/Sr <0.6, δ18O and trace element data indicate that many 87Sr/86Sr ratios are unaltered since diagenesis despite greenschist- to amphibolite-facies metamorphism, consistent with the documented behaviour of Sr and O during metamorphic fluid–rock interaction. Thus, the 87Sr/86Sr data are interpreted largely to reflect 87Sr/86Sr of coeval seawater. Currently available data show that Neoproterozoic seawater 87Sr/86Sr rose from c. 0.7052 at 850–900 Ma to c. 0.7085 or higher in the latest Neoproterozoic. Temporal changes at c. 800 Ma and c. 600 Ma bracket the range in 87Sr/86Sr values of calcite in Grampian, Appin and lowest Argyll Group (c. 0.7064–0.7072) and middle and uppermost Argyll Group (c. 0.7082–0.7095) limestones, consistent with a rise in seawater 87Sr/86Sr around 600 Ma. 87Sr/86Sr data are consistent with the sedimentary affinity of the Islay Subgroup with the underlying Appin Group, and with a possible time interval between deposition of Islay and Easdale Subgroup rocks. They indicate that the Dalradian, as a whole, is younger than c. 800 Ma.