We record the first onshore late Tertiary fossiliferous marine deposit from the 8000-km-long coastline of East Antarctica. Glaciomarine sediments containing abundant diatoms and molluscs were deposited in shallow waters off an open coast during the early Pliocene (∼4.5-3.5 Ma) and are now exposed less than 15 m above sea level on Marine Plain in the Vestfold Hills (lat 68°35'S,long 78°00'E). Marine diatoms suggest early Pliocene as the most probable age of the sediments. Fossil sand wedges show that the deposit was above sea level at least briefly before it was covered with water and subsequently capped with glaciomarine till. The coastline at the time of deposition of the marine sediments is inferred to have been close to but farther inland than that of today. The ice margin may have been ∼50 km farther inland, considerably reducing the ice volume of Antarctica. Climate at the time of deposition was interglacial and perhaps warmer than the present.