Neodymium and high-precision lead isotopic data are presented for Paleogene East Greenland flood basalts that erupted during an early phase of magmatic activity associated with the Iceland hotspot. The 6-km-thick volcanic sequence shows marked chemostratigraphic variations: lavas in the lower half of the sequence (Milne Land and Geikie Plateau Formations) have low 206Pb/204Pb values (17.8–18.4), abruptly changing to high 206Pb/ 204Pb values (18.8–19.3) in the overlying Rømer Fjord Formation, followed by intermediate 206Pb/204Pb values (18.6–18.8) in the uppermost Skrænterne Formation. These three isotopic groups of crustally uncontaminated lavas are broadly similar to spatially distinct isotopic domains found in present-day Iceland. The East Greenland data indicate that the same mantle domains present beneath Iceland today were present in the ancestral Iceland hotspot at 55 Ma, and were sequentially tapped during continental break-up as the spatially zoned mantle interacted with the rifting continental margin. The compositional domains within the Iceland hotspot appear to be long-lived features that, given estimates of Icelandic mantle-upwelling velocities, have vertical length-scales of at least ∼500 km.