<jats:p>Recent modeling suggests that changes in Southern Ocean sea‐ice extent potentially regulated the exchange of CO<jats:sub>2</jats:sub>release between the ocean and atmosphere during glacials. Unfortunately, a lack of high‐resolution sea‐ice records from the Southern Ocean has prevented detailed testing of these model‐based hypotheses with field data. Here we present high‐resolution records of Southern Ocean sea‐ice, for the period 35–15 cal ka BP, derived from diatom assemblages measured in three glacial sediment cores forming an ∼8° transect across the Scotia Sea, southwest Atlantic. Chronological control was achieved through a novel combination of diatom abundance stratigraphy, relative geomagnetic paleointensity data, and down‐core magnetic susceptibility and ice core dust correlation. Results showed that the winter sea‐ice edge reached its maximum northward extent of ∼53°S, at least 3° north of its modern limit, between ∼25 and ∼23.5 cal ka BP, predating the Last Glacial Maximum (LGM). Maximum northward expansion of the summer sea‐ice edge also pre‐dated the LGM, advancing to at least 61°S, and possibly as far north as 55°S between ∼31 and ∼23.5 cal ka BP, a ∼12° advance from its modern position. A clear shift in the seasonal sea‐ice zone is evident following summer sea‐ice edge retreat at ∼23.5 cal ka BP, potentially related to austral insolation forcing. This resulted in an expanded seasonal sea‐ice zone between ∼22.5 cal ka BP and deglaciation. Our field data confirm that Southern Ocean sea‐ice had the physical potential to influence the carbon cycle both as a physical barrier and more importantly through the suppression of vertical mixing and cycling of pre‐formed nutrients. Our data indicates that Southern Ocean sea‐ice was most effective as a physical barrier between ∼31 and ∼23.5 cal ka BP and as a mechanism capable of reducing vertical mixing between ∼22.5 cal ka BP and deglaciation. However, poor correlations with atmospheric CO<jats:sub>2</jats:sub> variability recorded in ice cores, particularly the lack of a CO<jats:sub>2</jats:sub>response during a rapid sea‐ice meltback event, recorded at our study sites at the same time as Antarctic Isotopic Maximum 2, suggest that Southern Ocean sea‐ice in the Scotia Sea did not play a controlling role in atmospheric CO<jats:sub>2</jats:sub> variation during the glacial.</jats:p>