The Eocene-Oligocene Transition (EOT; ~34.4–33.7 Ma) was a major shift in Earth’s long-term climatic evolution, marking the cooling from the early Paleogene greenhouse to the icehouse regime that has prevailed from the Oligocene until today. However, it remains uncertain which landmasses were already covered by ice sheets during the Early Oligocene Glacial Maximum (EOGM; ~33.7–33.2 Ma), an interval of peak glaciation immediately following the EOT. The scarcity of earliest Oligocene climate records in both Arctic and Antarctic regions hitherto prevented the reconstruction of environmental conditions and ice-sheet extent during the EOGM. Such constraints, however, are critical for assessing ice–ocean–atmosphere interactions during the early stages of the Cenozoic icehouse. Here, we present the first shallow-marine drill-core record of earliest Oligocene environmental conditions in West Antarctica’s Pacific sector. It comprises marine mudstones documenting the presence of a cool-temperate Nothofagus-dominated forest situated within a marine archipelago at 73.5°S palaeolatitude. Any evidence for marine-terminating glaciers is lacking, thus no land-based ice or only small ice caps existed in West Antarctica during the EOGM. Our new EOGM temperature and topographical constraints allow for more reliable verification of a fully-coupled Earth System Model. It simulates a large East Antarctic ice sheet already during the EOGM. However, West Antarctica does not glaciate until ~26 Ma, thereby illustrating the significance of asymmetric Antarctic ice sheet response during initial Cenozoic glaciation and highlighting the importance of differential regional response for future cryospheric change.