Using field observations followed by petrological, geochemical, geochronological, and geophysical data we infer the presence of a previously unknown Miocene subglacial volcanic center ~230 km from the South Pole. Evidence of volcanism is from boulders of olivine-bearing amygdaloidal/vesicular basalt and hyaloclastite deposited in a moraine in the southern Transantarctic Mountains. 40Ar/39Ar ages from five specimens plus U-Pb ages of detrital zircon from glacial till indicate igneous activity 25-17 Ma. The likely source of the volcanism is a circular -735 nT magnetic anomaly 60 km upflow from the sampling site. Subaqueous textures of the volcanics indicate eruption beneath ice or into water at the margin of an ice mass during the early Miocene. These rocks record the southernmost Cenozoic volcanism in Antarctica and expand the known extent of the oldest lavas associated with West Antarctic rift system. They may be an expression of lithospheric foundering beneath the southern Transantarctic Mountains.