Abstract Fogs over polluted regions often exhibit extremely low supersaturation, yet the controlling mechanisms remain poorly constrained. Existing frameworks attribute supersaturation adjustment mainly to competition among activated droplets, while the role of interstitial aerosol water has been largely overlooked due to observational limitations. Here, we develop a new approach based on an advanced optical instrument to achieve online, size‐resolved quantification of interstitial aerosol water uptake. By combining optical measurements with a machine learning–constrained framework, we retrieve dry aerosol volume and hygroscopic growth at saturation of interstitial aerosols, enabling fog water budget analysis. Observations on the North China Plain show that interstitial aerosol water contributes >50% of total fog water during formation and remains comparable (>25%) to droplet water throughout evolution. This competition helps suppress the supersaturation to <0.03%, limiting aerosol activation and droplet growth. These results identify interstitial aerosol water as a key regulator of fog supersaturation in polluted atmospheres.