Abstract Historically, polar marine phytoplankton were thought to primarily grow after the seasonal breakup of sea ice, when there is plentiful light available in the surface ocean. However, observations of substantial productivity under sea ice has called this assumption into question. Using a global Earth system model, we quantify under‐ice phytoplankton productivity in the Arctic and Southern Oceans. We find that phytoplankton growing under sea ice, which are invisible to remote sensing‐derived estimates of productivity, generate ∼100 Tg C yr−1 in each polar region. Additionally, while the sea ice conditions that permit under‐ice growth differ between the polar regions, in both poles the under‐ice blooms shift polewards and decline in importance near the end of the 21st century as sea ice loss accelerates. These changes likely have implications for food availability for the benthic and pelagic consumers in polar ecosystems and for global carbon cycling.