We present the first Antarctic terrestrial record of climate variations through the Cenozoic, based on the hydrogen isotope composition of hydrothermally altered minerals of intrusive rocks. This new record provides an independent geochemical proxy for continental climatic conditions; whereas, most land surface temperature proxies are biological. The temperature record is consistent with the range predicted by global climate models and proxy records for glacial and pre-glacial conditions in the Ross Sea region of Antarctica. The combined stable isotope (O and H) and age (40Ar-39Ar) determinations of hydrous mineral from Cenozoic igneous plutons and dikes show that the protracted time scale of magmatic activity and extensive hydrothermal exchange with local meteoric waters has preserved a semiquantitative climate signal of intervals in which atmospheric temperatures significantly fluctuated. These data also reveal that glacial episodes comparable with current polar conditions occurred repeatedly prior to geographical and thermal isolation of the Antarctic continent.