Global models predict that Antarctica has little suitable habitat for macroalgae and that Antarctic macroalgae therefore make a negligible contribution to global carbon fixation. However, shallow coastal surveys are rare at southern polar latitudes (beyond 71° S), and we here report diverse and abundant macroalgal assemblages in previously un-navigated coastal habitats of the Ross Sea near 72˚ S. We found significant macroalgal assemblages living at depths (>70 m) where they are rarely seen in temperate regions. Using global light modelling and published photosynthetic rates we estimate that Antarctic macroalgae may contribute between 0.9 – 2.8 % of the estimated global macroalgal carbon fixation. Combined, these factors suggest that Antarctic macroalgae may be a greater contributor to global carbon fixation and sequestration than previously thought, and that the vulnerability of coastal environments to climate change, especially sea ice extent and persistence, should be considered in Southern Ocean carbon budgets.