The first opening of the Bering Strait was an important palaeogeographical and biogeographical event for marine and terrestrial biotas in Asia and North America1,2,3, and an oceanographic event of global importance4,5,6. This event, however, has never been precisely dated, so it has not been accurately incorporated into models of global biogeography and oceanography. A recent find of the bivalve mollusc Astarte in southern Alaska is a clear signal that the strait had opened by at least the Late Miocene or earliest Pliocene epochs, because this genus otherwise dwelled only in the Arctic and North Atlantic oceans at that time1,2. Here we show that marine diatoms with Astarte are correlative with subzone b of the Neodenticula kamtschatica zone of the North Pacific diatom biochronology, yielding an age range of 4.8–5.5 Myr (refs 7, 8). Stratigraphic information suggests that this may be a minimum age range for the strait's first opening, which evidently occurred between 4.8 and 7.3–7.4 Myr ago. Our results contrast with past studies9,10,11,12,13 that suggested an age of 3.1–4.1 Myr for the initial opening of the Bering Strait.