The extent and thickness of Antarctic sea ice have important climatic effects on radiation balance, energy transfer between the atmosphere and ocean, and moisture availability. This paper explores the role of sea ice and related feedbacks in the Cenozoic evolution of Antarctic climate and ice sheets, using a numerical climate model with explicit, dynamical representations of sea ice and continental ice sheets. In a scenario of decreasing Cenozoic greenhouse gas concentrations, our model initiates continental glaciation before any significant sea ice forms around the continent. Once variable ice sheets are established, seasonal sea ice distribution is highly sensitive to orbital forcing and ice sheet geometry via the ice sheet’s control on regional temperature and lowlevel winds. Although the expansion of sea ice has significant climatic effects near the coast, it has only minimal effects in the continental interior and on the size of the ice sheet. Therefore the Cenozoic appearance of Antarctic sea ice was primarily a response to the growth of grounded ice sheets and was not a critical factor in episodes of Paleogene and Neogene glaciation. The influence of the East Antarctic Ice Sheet on sea ice, Southern Ocean surface temperatures, and winds has important implications for ocean circulation, the marine carbon cycle, and the development of the West Antarctic Ice Sheet. The sensitivity of sea ice to grounded ice sheets implies reconstructions of sea ice based on marine diatoms are good indicators of glacial conditions in the continental interior and may provide insight into the long-term stability of Antarctic Ice Sheets.