Reconstructed in-situ sea water temperatures in the East Atlantic for 1353 to 2006 from samples collected at Loch Sween Scotland (56°01.99'N, 005°36.13'W). dataset https://portal.medin.org.uk/portal/start.php?tpc=006_278d62189876538ea57e893261497158 To understand seasonal climatic variability in the North East Atlantic, a fortnightly resolution marine climate record from 1353–2006 was constructed for shallow inshore waters on the west coast of Scotland using red coralline algae. The data are available in an Excel file as mean winter and summer temperatures with 95% confidence intervals for each year from 1353 to 2006. SCUBA was used to collect a 46 cm core from a coralline algal (Lithothamnion glaciale) deposit in Loch Sween, Scotland. The core was frozen and sectioned longitudinally and into 2 cm horizons. Coralline algae from each horizon were sectioned along the length of each thallus. Mg, Ca, and Sr were quantified along each thallus using electron microprobe analysis. For the live collected surface specimens, this process enabled absolute dates to be assigned to each year’s growth band present within the coralline algae. Five thalli down core were selected for radiocarbon rangefinder dating at the Scottish Universities Environmental Research Centre. Live thalli and the five rangefinder thalli were used as anchor points in construction of a combined chronology which was fine-tuned using dendrochnological techniques. Twenty seven (including anchors) Mg/Ca time series were available; each from an individual thallus. The work was funded by the Natural Environmental Research Council and the Royal Society of Edinburgh. 20171003 20131125