Radiogenic isotopes in oceanic basalts provide a window into the different geochemical components defining the composition of Earth's mantle. Here we report the discovery of a novel geochemical signature in volcanic glasses sampled at a sub-kilometre scale along the East Pacific Rise between 15°37'N and 15°47'N. The most striking aspect of this signature is its unradiogenic lead ((206)Pb/(204)Pb=17.49, (207)Pb/(204)Pb=15.46 and (208)Pb/(204)Pb=36.83). In conjunction with enriched Sr, Nd and Hf signatures, Pb isotopes depict mixing lines that trend away from any known mantle end-members. We suggest that this unradiogenic lead component sampled by magmatic melts corresponds to a novel upper mantle reservoir that should be considered in the Pb isotope budget of the bulk silicate Earth. Major, trace element and isotope compositions are suggestive of an ancient and lower continental origin for this unradiogenic lead component, possibly sulphide-bearing pyroxenites that were preserved even after prolonged stirring within the ambient upper mantle.