The Southern Ocean has emerged as a key region for constraining aerosol‐climate interactions due to its relatively low anthropogenic influence. Sulfate is an important aerosol over the Southern Ocean, and models suggest dimethyl sulfide (DMS) is the largest source of sulfate during summer. However, sulfur isotopes of sulfate (δ³⁴S(SO₄²⁻)) in Antarctic ice cores suggest a significant contribution from a previously unexplained non‐DMS source. Here we show that the fractional contribution from passive volcanic degassing (f_volc) explains observed δ³⁴S(SO₄²⁻), and that a global chemical transport model underestimatesf_volcacross Antarctica. Underestimatedf_volcimplies that the model mischaracterizes sulfate sources in this important region. The discrepancy between observed and modeled sulfate sources can be reconciled by increasing passive volcanic sulfur degassing emissions and decreasing DMS emissions. Our results imply that current biases in emissions inventories could bias assessments of aerosol‐cloud interactions in the Southern Ocean region and globally.