The ‘greenhouse climate’ of the Late Cretaceous epoch was one of the Earth’s warmest periods of the past 140 Ma, particularly at high latitudes. However, records allowing insights into terrestrial environmental conditions south of the Antarctic circle during that time are extremely rare. Hence, it remains highly elusive how the sensitive South Polar environment may have been impacted by such an extreme climate. Here we report a unique sedimentary sequence that was recovered with the MeBo-70 sea floor drill rig from the central Amundsen Sea Embayment shelf, West Antarctica. The record contains ~26 m of quartzitic sandstone underlain by a lithified swamp deposit that consists of a ~2 m-long complex and intact network of in-situ fossil plant roots embedded in a mudstone matrix. The lower ~1.5 m of this mudstone contain a highly diverse pollen and spore assemblage, documenting a temperate coastal lowland rain forest environment with mean annual temperatures of 11-15°C at a palaeolatitude of 77°S. Hence, the drill record provides the hitherto southernmost evidence of Cretaceous terrestrial environmental conditions and reveals a ‘greenhouse climate’ that was capable of maintaining a temperate environment much further south than previously known. The predictive capabilities of model simulations for high-latitude climate and environment characteristics for this critical period of Earth’s climatic history can therefore now be evaluated more reliably.