The Cenozoic cooling that occurred over the past 50 Ma is accompanied by an increase of Mg/Ca ratio in seawater. How this change in seawater chemistry is linked to climate change is still disputed. Mg isotope ratios of seawater could distinguish several possible causes including dolomitization, authigenic clay formation and changes in rates of silicate- and carbonate weathering. Former reconstructions of Mg isotope ratios of paleo-seawater are based on foraminifera, corals or carbonate muds, which however yield conflicting results. Here we assess the suitability of the bivalve Glycymeris as an archive for paleo-seawater δ26Mg (the standardized 26Mg/24Mg ratio). Their potential as geochemical archive advantage arises from their strong evolutionary conservatism, thick shells and a fossil record dating back to the Early Cretaceous. We report Mg isotope signatures of shells of three recent Glycymeris species from the Adriatic Sea that show an increasing fractionation with increasing ontogenetic age, a property that we use to determine δ26Mg of paleo-seawater from fossil Glycymeris specimens.