Introduction: Environmental, Social, and Governance (ESG) uncertainty has emerged as an important source of financial risk, particularly for pollution‐intensive industries that are highly exposed to sustainability‐related policy and regulatory changes. However, evidence on how ESG uncertainty is associated with sectoral volatility remains limited. This study examines the relationship between the U.S. ESG Uncertainty Index (U.S. ESGUI) and the volatility dynamics of the six S&P 500 sectors with the highest historical TRI‐based pollution rankings: Energy, Utilities, Consumer Discretionary, Consumer Staples, Materials, and Industrials. Methods: The study employs the GARCH–MIDAS framework, which integrates high‐frequency daily stock returns with the low‐frequency U.S. ESGUI to capture short‐term volatility dynamics and long‐term ESG‐related effects. Forecasting performance is evaluated against GARCH(1,1), EGARCH(1,1), and RV‐GARCH–MIDAS benchmark models. Diebold–Mariano (DM) tests are used to compare predictive performance, and additional crisis‐period out‐of‐sample analyses examine forecasting performance during the COVID‐19 pandemic and the Russia–Ukraine conflict. Results: The results reveal that U.S. ESG uncertainty is significantly associated with long‐run volatility across all six sectors, while short‐run volatility remains highly persistent. Sectoral heterogeneity is evident: Energy, Utilities, Consumer Discretionary, and Materials exhibit negative long‐run associations with U.S. ESG uncertainty, whereas Consumer Staples and Industrials exhibit positive long-run associations. The ESGUI‐GARCH–MIDAS model generally achieves lower forecast losses than the GARCH(1,1), EGARCH(1,1), and RV‐GARCH–MIDAS benchmark models across the six sectors. DM tests further show that the ESGUI‐GARCH–MIDAS model significantly outperforms the RV‐GARCH–MIDAS benchmark under both the TSR and TSQ criteria across all six sectors (p < 0.001), indicating that the forecasting gains are not attributable solely to the mixed‐frequency framework. Compared with GARCH(1,1) and EGARCH(1,1), the DM results provide stronger and more consistent evidence under the TSR criterion, although relative forecasting performance varies across loss functions and sectors. The additional crisis‐period out‐of‐sample analyses provide further evidence on the model’s forecasting performance during periods of heightened market uncertainty. Discussion: The findings suggest that the U.S. ESGUI contains useful information for understanding and forecasting sectoral volatility, while its association with long‐run volatility varies across the most polluted sectors. The study contributes to the growing ESG–finance literature by highlighting the relevance of sustainability‐related uncertainty in sector‐level volatility modelling and forecasting.