The chemical structures of dissolved organic compounds in natural waters, including the degree of aromaticity, affect their physical, chemical, and biological properties and ultimately the fate of carbon in aquatic systems and during water treatment. Herein, a new fluorescence-based aromaticity index named ARIX is shown to link the composition of aquatic dissolved organic matter to its aromaticity across diverse aquatic systems in both bulk DOM and extracts. ARIX predicts SUVA, a widely used proxy of aromaticity, more accurately than the prevailing optical indices. It also predicts the percentage of polycyclic aromatic and polyphenolic molecular formulas determined by FT-ICR MS and the ratio of "humic substances" to "building blocks" fractions determined by LC-OCD, indicating that it is additionally a proxy of DOM molecular weight. In waterbodies exhibiting decoupling between DOC and absorbance linked to biogeochemical processing, DOC concentrations are more accurately predicted by using a multilinear model to account for interactions between light absorption and aromaticity. The results deliver new insights into widely discussed trends in DOM optical properties and the molecular structures underlying optical measurements in the aquatic milieu. They further represent an important step toward improved real-time monitoring of DOC concentration, reactivity, and fate.