Here we report on mid-Oligocene globally synchronous Arctic dinoflagellate migration events, calibrated against chron C9n. We show that sudden appearances and marked abundance increases of the Arctic taxon Svalbardella at lower and middle latitudes coincide with the Oi-2b benthic δ18O glacial episode, dated as ca. 27.1 Ma. These unprecedented migrations are taken to indicate anomalously strong surface-water cooling during Oi-2b time, in turn associated with strong concomitant Antarctic ice-sheet growth and sea-level lowering. We estimate the duration of these unique Svalbardella migrations and the associated episode of profound cooling as ∼500 k.y. Our records suggest a close link between this distinct Oligocene glaciation episode, strong sea-level fall, and the classic lower-upper Oligocene, or Rupelian-Chattian, boundary, dating this boundary as ca. 27.1 Ma.