Abstract Earth system models exhibit persistent biases in simulating cloud condensation nuclei (CCN) over the Southern Ocean and Antarctica, largely due to inadequate representation of aerosol sources. Here, we present year‐round (January 2023–February 2024) measurements of aerosol hygroscopicity (HTDMA), particle number size distribution (SMPS), and CCN concentration from King Sejong Station (KSJ), Antarctica, to constrain CCN sources in this pristine environment. Size‐resolved hygroscopicity‐based source apportionment shows that sea salt aerosol (SSA) contributes 43%–54% of CCN at cloud‐relevant supersaturations (Sc = 0.2%–1.0%). Compared against the Copernicus Atmosphere Monitoring Service reanalysis (CAMSRA), our observations demonstrate that models underestimate SSA‐derived CCN at KSJ by a factor of 30 at Sc = 0.2%, providing critical constraints for aerosol–cloud interactions.