New constraints on the age of the Cockburn Island Formation, northern Antarctic Peninsula, resulting from whole rock laser-stepped heated 40Ar–39Ar dating of associated basalt and palaeomagnetic re-calibration of the ranges of the formation’s fossil diatom taxa, suggest that interglacial conditions existed around 3 million years ago. The refined age of the deposit supports continent-wide Late Pliocene warming in Antarctica, and makes more likely the occurrence of extensive marine incursions in East Antarctica at that time.