Isotope (δ18O) and minor element (Mg/Ca, Sr/Ca) analyses of a fossil shell of the marine mussel Mytilus trossulus (Gould) from the Whisky Run terrace at Coquille Point, Bandon, Oregon, enable reconstruction of a paleoclimatic profile for the temperate northeastern Pacific ca. 80–85 ka. The Whisky Run terrace is correlative with marine oxygen isotope (δ18O) substage 5a, the final sea-level highstand of the last interglacial complex. Herein we provide sea-surface temperature estimates and report on a new technique that allows estimates to be made of δ18O of seawater and riverine discharge from the skeletal chemistry of a single mussel shell. Estimates of mean annual temperature from Mg/Ca ratios are lower than those suggested by the accompanying fossil fauna, and significantly lower than present-day conditions. The data also suggest a decrease in the range of seasonal temperature extremes at 80 ka compared to today. Estimates of the fresh-water and seawater δ18O end members, −4.5‰ and +0.5‰, respectively, are slightly greater than modern values.