<span style="font-weight: 400;">The Southern Ocean plays a vital role in regulating Earth’s climate by absorbing large quantities of carbon dioxide, but its productivity is strongly limited by the availability of iron needed for phytoplankton growth. Sea ice is a crucial seasonal reservoir of iron; however, the processes responsible for its enrichment remain poorly understood. Here we use a process-based model to show that frazil ice formation, a common feature of winter sea-ice growth in the Southern Ocean, can scavenge dissolved iron from seawater and concentrate it in newly formed ice. First-order estimates scaled to the Antarctic sea-ice zone suggest that frazil-origin ice could supply ~16–33% of the seasonal dissolved iron pool in the upper 10 m. Because this iron is released as a short-lived but intense pulse upon melt, frazil ice provides a pathway that can trigger phytoplankton blooms, linking polar sea-ice processes directly to global carbon cycling.</span>