Sea-ice extent is predicted to decrease in a warming climate. However, despite global warming over the past century, total Antarctic sea ice remained relatively stable from 1979 until 2015, before strongly melting. Here we explore the long-term sea ice variability by reconstructing Antarctic sea ice since 1700 CE, based on paleoclimate records and data assimilation. Our results indicate a decline in southern hemisphere sea-ice extent over the 20th century, driven by a reduction of 0.26 million km² in the Weddell Sea that reached values at the end of the century lower than any other reconstructed period. The Ross Sea experienced an increasing sea-ice cover trend due to a low-pressure system located off the Amundsen Sea coast, offset by a decreasing trend in the Bellingshausen-Amundsen Sea. Models failed to account for the Ross Sea increase, resulting in an overly uniform estimate of Antarctic sea ice loss over the 20th century.