The Late Cretaceous is considered to have been a time of greenhouse climates, although evidence from Maastrichtian sediments for rapid and significant sea-level changes suggests that ice sheets were growing and decaying on Antarctica at that time. There is no direct geological evidence for glaciation, but we present palynomorph records from Seymour Island, Antarctica, that may suggest Maastrichtian sea ice. The dinoflagellate cyst Impletosphaeridium clavus is dominant. We propose that its profusion may signify the accumulation of resting cysts from dinoflagellate blooms related to winter sea ice decay. Prior to the Cretaceous-Paleogene transition, I. clavus decreased dramatically in abundance; we link this with climate warming. Terrestrial conditions inferred from pollen and spore data are consistent with our climate interpretations based on I. clavus together with δ18O values from macrofossils. These data and our interpretation support the presence of ephemeral ice sheets on Antarctica during the latest Cretaceous, highlighting the extreme sensitivity of this region to global climate change.