Reconstructions of seawater Mg isotopic composition (δ26Mg) can provide novel insights into the processes that control the major ion chemistry of seawater over geologic time scales. A key period of interest is the Cenozoic (ca. 65 Ma to today), during which the Mg/Ca ratio of seawater increased by a factor of 2–3. However, two published records of seawater δ26Mg over the Cenozoic disagree, making it difficult to draw conclusions about mechanisms driving seawater Mg/Ca change over the past 65 m.y. Here we present a new record of seawater δ26Mg from a set of well-preserved fossil corals, ranging in age from Paleocene to Recent. Fossil coral δ26Mg decreases by ∼0.3‰ between the early Cenozoic and the Oligocene, then increases by ∼0.15‰ between the Oligocene and present, in strong agreement with the published record derived from bulk pelagic carbonate. Together with this existing record, our fossil coral data suggest that the rise in [Mg]seawater over the Cenozoic was mainly driven by an increase in Mg silicate weathering or a decline in Mg uptake in marine silicates. In contrast, we suggest that changes in the rate of carbonate weathering and dolomite formation likely played a minor, but not insignificant, role in the global Mg cycle over the Cenozoic.